Hey, I'm not trying to come off as disparaging at all and I'm glad that you're experimenting and finding some success with lower tier Pokemon like Sliggoo, but I wanted to just comment on this line of reasoning. Being able to switch into and defensively check some prominent offensive threats unfortunately is not enough to make a Pokemon good or viable if that Pokemon can't do anything back to threaten what they switched in on or make progress in some other way. Compare Chansey in SM/Blissey in SS to them in SV. I remember z0mOG (one of the best SM players ever, although I'm admittedly a little biased since he's also my personal favorite player ever) saying before that Chansey is one of the most threatening Pokemon in SM because it was so difficult to safely switch in on Toxic and Seismic Toss and it made progress easily with Stealth Rock. Even though Blissey's viability in SS dwindled a little throughout the generation, it remained in OU, and its viability was greatly bolstered by being able to Teleport out to regain momentum after switching in on whatever Special Attacker that it did. However, in SV Blissey has fallen all the way down to NU, even though it's just as good as ever at sponging hits. The problem with Blissey in SV is that without Teleport, it can't regain momentum and without Toxic, it has a hard time preventing setup sweepers from abusing it. Sure, Blissey has still seen some good usage on stall teams in OU, but stall teams don't often care as much about needing to make progress since they can win games by attempting to outlast opposing teams without really threatening them back (I'm not as familiar with SV as a whole and particularly stall in SV though, so I'm not sure if stall teams have had to adapt their approach more due to the nerfing of the PP of recovery moves).
I'm not sure if you were using Sliggoo on a stall team, but if you weren't, I think that the problem with it, and other similar Pokemon that offer nice defensive calcs in a vacuum, is that they can't reliably contribute much towards wining a game. This is an extreme example, but look at Shedinja in previous generations. It completely blanked some of the most threatening Pokemon in even Ubers (of course considering you've dealt with hazards and Sand/Hail), but it has historically been rarely seen because it's very limited in terms of how effective it can be after switching in. I've personally always thought that Shedinja stall teams could be good in SM and SS and I remember
Tricking winning a World Cup game with Shedinja stall back in SS, but again, stall teams can be different. Not to belabor this point further, but compare Shedinja to other defensive Pokemon like Toxapex or Specially Defensive Heatran in SM and SS. While Shedinja, Pex, and Tran can all switch into threatening attackers, so many more Pokemon can easily switch into something like Shedinja than they could Pex or Tran. The combination of Scald, Toxic, and Knock Off made Pex notoriously difficult to safely switch into, and the same can be said about Tran with Magma Storm, Toxic, Taunt, Lava Plume, and even making additional progress with Rocks. To bring this back to SV, compare Sliggoo to something like Galarian Slowking. If Slowking-G switches into something that it can't really threaten back, it can just Chilly Reception out to safely bring in a teammate that can be offensively threatening on their behalf (and Slowking can sometimes also afford to use Future Sight first, to create an even more threatening scenario for the opponent). If Sliggoo switches in and successfully takes a hit, great, but if it can't do much to threaten the opposing team back, you aren't really in a much better situation; if you need to in turn switch your Sliggoo out of whatever switches into it, without Sliggoo having done much in return, then you're back to where you started with needing to switch in Sliggoo in the first place.
Again, hopefully I didn't come off aggressively or dismissive in any way, since that wasn't at all my intent; I just decided to write this because I've seen a similar sentiment in the past espoused regarding other Pokemon that are defensively good on paper but don't function well in practice. I have seen more "random," not fully evolved Pokemon with Eviolite work in the past over their evolutions, but that was because they offered some very specific niche. The main example that I'm thinking of is that Ben Gay/Craing, the greatest team builder ever, used Trubbish as a Spikes setter at one point in SS, as seen by
three of the teams in this team dump he posted (Untitled 27-29), since Trubbish retained Sticky Hold while Garbodor did not. Knock Off was rampant at that time so you could conceivably argue that having Sticky Hold was a big enough selling point to use Trubbish, especially since it ensured that Trubbish would be able to keep its Eviolite and remain relatively bulky, while also having Spikes/Toxic Spikes, Toxic, and Corrosive Gas to make progress and cripple switch-ins. However, with all of that being said, Trubbish was still obviously flawed and never was very relevant in the meta - I only really remember ima bringing one of Ben Gay's teams with it to a
Smogon Tour Playoffs game, which he unfortunately lost.
So, to summarize what I wrote, I feel like it's cool trying to experiment with lower tier/NFE defensive Pokemon but the problem that these types of Pokemon tend to run into, and what separates them from higher tier defensive options, is that if a Pokemon can't reliably make progress or be threatening in some way after switching in, it doesn't matter much how well it can take hits.