1) If a "Noob" does not have a certain pokemon and set, I do not see what the difference would be between them going to catch it in a dungeon vs. going to catch it in another location to get the right ability, I admit it is minorly less convenient than getting a regular ability poke, but it's not going to be hard to purchase a poke with its HA off GTL like you do now.
2) Rebreeding does not exist "Just to stay relevant", it is the primary money sink in the game that helps prevent inflation, it is what value is tied to and by that I mean if we have hidden abilities added and simply tied to Ability Pills then a pokemon with a HA and a pokemon with no HA will be the same value, and a lower value due to the supply being higher for both.
It does, on top of this, help giving you future projects to do even if you have no current pokemon you have an interest for.
I genuinely see the complaint that you have to rebreed a pokemon with it's hidden ability akin to complaining you have to breed a garchomp from scratch when it is added and can't just "Ability pill" a garchomp into existence. they fill virtually the same role in terms of longevity.
3) It's incredibly unlikely that a person would have literally every pokemon in the game, you're correct, however most players aren't interested in 90% of the game's pokemon, however there's a high chance they'd be interested in a specific HA version of a pokemon, so in the case where they never made the original pokemon as they had no interest then it isn't rebreeding but at the same time is an example of how it can add longevity as it can add interest in a pokemon that you had no interest in before.
4) "people" please remember that minorities often have the loudest voice, i'd imagine a very large part of the player base doesn't particularly mind this philosophy however the people who do mind ofc are going to make the most comments on the forums. keep this in mind before just assuming that most of the playerbase is in agreement either way on this debate.
Also, You know that sometimes what the players want in the short term is bad for the game long-term right? you can be snarky and insulting with your impersonation of the response all you want, but it's incredibly likely that they did consider this and decided after careful consideration that it is probably not a good idea. Also no one is forcing you to make these HA pokemon, you do what you want because you want it enough, clearly you want the HA pokemon if you are willing to write essays and circlejerk about it on the forums (at least i'd hope) so why are you so opposed to investing some of your money / time in game, which it wouldn't even take that much more than a normal pokemon, to make a new HA poke? It's such a minor lazy issue I can't personally empathize with your complaints.
5) I dunno what you're talking about that shit on long term players, also lmao the game has never been so dead, Unova brought a lot of players back and a lot of them are still waiting in the wings, as soon as dungeons are added they'll come back again, times after updates will be a spike in players followed by a gradual decline till next update.
If you're referring to the genderless pokemon breeding change as shitting on old players I will agree it crashed a market, but the market was founded on the fact it was just asinine to breed your own pokemon for, and is a very similar kind of change you are asking for which is quality of life because of something that makes it difficult to breed a pokemon in terms of time and money, except genderless pokes were far FAR harder than this and definitely warranted the change (Unlike HA pokes imo)