Now bring out the fact that over 50% of Rillabooms run choice band, what is Hydrapple doing? Tera on a U-turn?
Also talking about Hydrapple as a rain check,
252+ Atk Choice Band Barraskewda Flip Turn vs. 248 HP / 0 Def Hydrapple in Rain: 53-63 (12.7 - 15.1%) -- possible 7HKO
252+ SpA Archaludon Draco Meteor vs. 248 HP / 192 SpD Assault Vest Hydrapple: 294-348 (70.8 - 83.8%) -- guaranteed 2HKO
So if hazards are up, it's a roll to just die right there. If Hazards isnt up then just hit it again. If its boots then it's dying to draco. If you tera then you're not longer a switch in to Barra.
Brother is not even a real rain check.
You didn't use a Band Calc in the comment I was responding to, so I wasn't addressing the Band Calc as part of your point there. Regardless
252+ Atk Choice Band Rillaboom U-turn vs. 248 HP / 0 Def Hydrapple: 226-266 (54.4 - 64%) -- guaranteed 2HKO after Grassy Terrain recovery
This isn't un-recoverable damage for a Regenerator mon that's getting a turn of Terrain healing if it stays to eat the U-Turn. It chunks but that's an amount of health you could see a mon eating without being crippled the rest of the game.
(And now for my trademark of writing an essay wall in response to a couple lines)
As for the Rain interaction, it's not as cut and dry as you think there. Barraskewda is 2HKO'd by Hydrapple's Earth Power, never mind the STAB moves, while having at best a 2HKO roll with Band Close Combat, which it should never click because the -1 makes an OHKO possible (93% chance with 1 Spike/SR worth of Chip). Fickle Beam similarly has a decent OHKO chance after a Spike.
56+ SpA Hydrapple Earth Power vs. 0 HP / 0 SpD Barraskewda: 152-179 (57.7 - 68%) -- guaranteed 2HKO
56+ SpA Hydrapple Earth Power vs. -1 0 HP / 0 SpD Barraskewda: 228-269 (86.6 - 102.2%) -- 93.8% chance to OHKO after 1 layer of Spikes
56+ SpA Hydrapple Fickle Beam vs. 0 HP / 0 SpD Barraskewda: 202-238 (76.8 - 90.4%) -- guaranteed 2HKO
So if Barraskewda has no good move besides Flip Turn, Archaludon's not coming in on a free. Hydrapple can afford to click Earth Power or its "safe" STAB option in Fickle Beam (and should because Archaludon's lack of recovery, offensive Snowballing, and specific defensive benefits to Rain make it Priority #1 to make progress against).
56+ SpA Hydrapple Earth Power vs. 204 HP / 0 SpD Assault Vest Archaludon: 166-196 (44.6 - 52.6%) -- guaranteed 2HKO after 1 layer of Spikes
56+ SpA Hydrapple Fickle Beam vs. 204 HP / 0 SpD Assault Vest Archaludon: 111-132 (29.8 - 35.4%) -- guaranteed 3HKO after 1 layer of Spikes
If there's a Hazard down, Archaludon, the trade machine, itself is playing prediction unless it also Tera Fairies. If Hydrapple goes Tera Fairy, Draco Meteor just wastes the turn, while Flash Cannon at best 2HKO's on a SE hit. +1 Body Press isn't worth clicking regardless, and Electro Shot counts on Hydrapple going Tera because the 4x Resist makes the Chip negligible (Flip Turn + Electro Shot is 25% damage, still leaving +1 SE Flash Cannon as a roll to KO). The incredibly simplified flow you presented above runs on the assumption Archaludon came in for free, on a fast U-Turn off a mon that doesn't threaten Hydrapple into not clicking those moves (i.e. Barraskewda not clicking Flip Turn and simply attacking doesn't put enough pressure to click Giga Drain instead).
So a pristine Arch coming into a spike has 2 Moves for Tera Hydrapple and 1 for non-Tera. It basically must Tera itself to avoid a 2HKO if it came in on Earth Power or Full-Power Fickle Beam (knowing Arch is in the back leaves Hydrapple little reason to click its Grass STAB into the fast Pivot mon that dies even to neutral damage), which is an ask to acknowledge (even if not a dealbreaker) given teammates have use for that as much as it does like Bolt or Water-Stacking Barraskewda. Hydrapple losing the "both Tera" scenario by either being KO'd by Arch or not checking Barraskewda also assumes that it's Tera Fairy as on the example set, when if you're running it specifically to strength the Rain match-up, also presents options like Grass-Stack (ditching Dragon Weaknesses for Mono-Grass has merit defensively) or Tera Water to resist potential Ice damage, the latter letting it sit on Pelipper at the cost of having to predict around Electro Shot more.
56+ SpA Tera Grass Hydrapple Giga Drain vs. 204 HP / 0 SpD Assault Vest Tera Fairy Archaludon: 138-164 (37 - 44%) -- guaranteed 3HKO (16.6 - 19.7% recovered)
252+ Atk Choice Band Tera Water Barraskewda Liquidation vs. 248 HP / 0 Def Tera Grass Hydrapple in Rain: 204-240 (49.1 - 57.8%) -- 96.9% chance to 2HKO
Grass stack also puts a dent in Tera Archaludon while allowing it to outheal anything besides un-debuffed Draco and Flash Cannon, making it a safe Tera to click defensively whether or not Arch goes for its own. If Archaludon switches out that's more progress on it than on Hydrapple given Regenerator, and others on Rain hate to go up against the Apple without Tera themselves (Raging Bolt 2HKO's at best while OHKO'd by Draco or 2HKO'd by EP + Fickle Beam without Fairy, Damp Rock Defensive Pelipper Hurricane can't 2HKO even with a layer of Spikes).
It's not exactly eating everything just by existing with a simple plan, but Hydrapple has a particular niche here in that Arch/Skewda doesn't run it over. the way most things worry about beating on and struggling/being cautious into the other (Clod, Walking Wake, Bolt, Kyurem).