Quick reminder on what sun did for Ho-Oh. It removed its water-type weakness, crippled Thunder, and heavily increased the damage output of Sacred Fire. This meant that, under sun, the only means to KO Ho-Oh was through strong physical attacks, Stone Edge, or Bolt Strike. (and that's assuming SR is in play) All of these users are very vulnerable to Sacred Fire which is likely to be used more often because of the boost. If you wanted to expand your options you needed to change the weather. These advantages compensated for those times where you were unable to remove SR when playing against more defensive teams.
You're overstating it. Water-weakness removal is only relevant for Palkia as Kyogre summons its own rain and no other common Ubers sweeper uses Water-type attacks. Likewise, the most common user of Thunder is Kyogre, which again summons its own rain, and Thunder is growing increasingly less common on Palkia (Toxic rest is new standard) and Dialga (usually runs Toxic), not to mention Ho-Oh has the raw bulk to tank unSTAB Thunder. The power boost to Sacred Fire, again, is only relevant on walls such as Lugia and Gliscor, who are both still shaky checks, and it is still a quite spammable move due to its 50% burn rate. Furthermore, most users of Stone Edge were Tyranitar (who has sand), Terrakion (who can't switch in anyway since it doesn't resist Brave Bird and sucks this gen lol) or Reshiram (who can't be burned and is quite uncommon).
Again, you are placing too much importance on sun and not enough on Ho-Oh itself. You are also acting like sun isn't ever going to be up this gen - it's certainly harder to make use of, but Groudon and Ho-Oh still have quite solid synergy with each other just on their own merits, so it still does get the chance to make use of sun.
Viable anti-SR measures in BW2: (as in, viable in reliably supporting Ho-Oh)
-Cloyster
-Sand Excadrill
-Kabutops
-sorta Magic Bounce
Viable anti-SR measures in XY:
-Cloyster
-Webs Excadrill
Forretress, Tentacruel, rain Excadrill, Defog Arceus, and other random Defog users are all either easy to pressure, easy to punish, or have difficulty in repeatedly removing Stealth Rock. Just because Support Arceus is bulky doesn't mean it can get around fast Taunt or afford to be switching in every time SR is up and Defogging, especially when it is already a momentum dropper. (and some SR setters, like Dialga, can switch in on the Defog and replace the hazard)
This comparison is laughable to me. First of all, you can go ahead and take Kabutops off that list since no one is running Ho-Oh with Kabutops (and if they are then that kills your sun argument). Magic Bounce was extremely unreliable in the first place since Espeon only worked once (and if your argument is consistency then you might as well not include Espeon) and Xatu could only stop defensive setters like Ferrothorn and falters to Dialga/offensive Groudon. Cloyster and Excadrill still work in this metagame, even Rain Excadrill since it sets Stealth Rock and can beat Dialga, Zekrom, and Klefki. Defog on top of that means we have MORE ways to keep hazards in check this gen.
And no, Defog Arceus doesn't suck. Water/Fairy Arceus softcheck a lot of Pokemon and usually find a lot of opportunities to come in. Ghost Arceus checks Mewtwo/Ekiller and can't be trapped, and gives free turns to few Pokemon thanks to its potent STAB. More niche types such as Rock and Grass can still find use on some teams. Arceus is also far bulkier/easier to fit into a team than any Spinner and it's ability to clear hazards cannot be stopped simply by using a Ghost like it was so easy to do in 5th gen. Giratina-O and Mega Scizor aren't as good at Defogging, but they still work well and are much easier to teambuild with than things like Cloyster or Magic Bounce.
And fast Taunt? The only common Taunt user faster than Arceus is Mewtwo. Does 120 Speed not mean anything to you?
All this to say that Ho-Oh was only superficially buffed in XY. In reality, the number of viable means to support it so that its advantages can be used are limited. It doesn't have as many reliable ways to handle Stealth Rock nor sunlight to compensate for the turns that the hazard is present.
Overall, Ho-Oh is still better in XY than in BW2, it's not merely superficial. Most of your arguments are stemming from the following faulty premises:
-Ho-Oh was some sort of demigod in BW2 compared to what it is now
-Ho-Oh relies on sun to be an offensive threat
-Ho-Oh requires too much support because it is x4 SR weak
-Defog is too inconsistent of a hazard control strategy to be viable
When in reality:
-Ho-Oh was great in BW2 but is honestly better now since Defog is harder to stop (not saying it can't be stopped at all but it is still better than relying exclusively on Spin) and it can fit in more teams due to more options for hazard control being present
-Ho-Oh does not need sun to be an offensive threat, sun is merely a bonus and Ho-Oh can work on other types of teams (like sand or even rain as teams that relied on sun tended to be Ho-Oh weak themselves, how bout them apples) besides. Ho-Oh also fits extremely well on defensive teams since it checks a ton of stuff.
-Ho-Oh is well worth hazard control support, which benefits not only it but the other Pokemon on its team, so its in no way overly specialized.
-Arceus is much more likely to get off a Defog than Forretress, Tops (who btw is not a consistent spinner since it loses to Giratina and is extremely frail), Cruel, or Drill getting a spin past Giratina or Ghostceus. If anything its way more consistent than Rapid Spin. Yeah I acknowledge that Defog isn't perfect, can be removed with Shadow Tag likes tons of other things, and that most Defoggers can be Toxiced and worn down but even one or two Defog uses in a game is better than not being able to spin at all unless your name is Cloyster...who is still viable. You can also run a cleric, of which there are much more viable users (limited to Blissey in Gen 5, but now in Gen 6 we have Xerneas, Sylveon, and Clefable all of which are useful in their own ways).
Some quick nitpicks:
-reading too far was directed at Haruno
-gg no re was directed at Heatran not getting a second chance after Earthquake
-Water Arceus was not an effective Ho-Oh check on sun, nor did it have good synergy on rain. It was only effective as a Ho-Oh check on sand, which had more difficulty in fighting the weather war. (Water Arceus can't check if sun is up)
-Roost Gliscor is a recent thing but it can do some impressive stuff, js
-Electric Arceus is only barely 2HKO'd by Sacred Fire while Judgment OHKOs after Stealth Rock. It can still switch in on Roost, Brave Bird, or Substitute. Earthquake hurts more than Sacred Fire but it's nowhere near a OHKO either. Burning Electric Arceus protects it from Toxic. It's not a perfect check but it's definitely a viable one and something that is fairly easy to fit onto a team.
-Lugiass doesn't mind burns much because that protects it from something it really hates: Toxic. It doesn't mind burns even less when you remember that you are using repeatedly using Defog to remove SR.
My bad on the first two, wasn't clear.
Waterceus could still tank Sacred Fire fairly well (doesn't 2HKO in Sun) and Toxicstall it, though yeah Judge won't do much in sun and I agree it doesn't work so well on rain teams. It still existed before though.
Roost Gliscor is probably cool but it also costs it a moveslot (which means you lose SR or Taunt which is a steep price to pay).
You don't think its sad Eleceus needs SR to OHKO Ho-Oh with a STAB super effective attack? It's better than most things but its a really squishy check.
Lugia still doesn't like being burned since it makes abusing Multiscale much harder and forces it to recover more. It does give it Toxic immunity but it's not like it suddenly has Magic Guard lol.
Not really lol.
That being said, I am fine with it dropping to A, but no more than that. A- is too low and your comparisons to how everything was better in Gen 5 are extremely flawed.