I'm not sure what the answer is
I don't think there should be ANY 5-month timegated grinds that's for starters lol.
it's a lot easier to just go hardcore in one or two modes rather than have to do a song and dance to convince the player-gated higher end content in higher end PvP, dungeons and raiding to give you a chance.
The big problem here is that MMORPGs have repeatedly allowed utterly toxic player-gated communities to form and continue. The first MMORPG to go this way, was, bizarrely, GW2, which fell almost immediately into this kind of toxicity (well, in less than two years from release). WoW lasted a hell of a lot longer, but by 2020 it was heading that way, and it's simply because Blizzard is willing to make content that players perceive as "hard" (in reality it tends to just be fiddly, annoying, require tons of rote memorization or to have a high gear requirement people don't like to be real about) and then lock all the major rewards behind that, then act Surprised Pikachu when people are scumbags about letting people even access that content.
I think the real solution is, perhaps counterintuitively to some people, to make group content in general easier and
more rewarding, and to focus on fun rather than difficulty or precision, and to have more difficult stuff which absolutely DOES NOT offer better rewards in terms of gear or "skins" (including mounts), but does offer titles and similar (maybe a recolour of a mount, but not a new mount). The game's peak popularity was always when it was, frankly, bloody easy i.e. Vanilla and WotLK - TBC, which was a lot tougher, saw massively less engagement with endgame content (Blizzard themselves said less than 0.5% of accounts with a max-level character in TBC even downed a single boss in the final raid, contrast with Vanilla where like 40% of accounts with a max-level character had killed most or all of the bosses in MC, and it was over 20% for BWL, but then sharply down to below 5% for original Naxx), and Cataclysm, which was a nightmare compared to WotLK, also saw a massive decline and then WoW's population crashed and has never recovered.
Blizzard live in this bizarre paranoid state when they believe people will finish everything and quit, but I know their metrics show absolutely nothing of the sort has ever happened (because it hasn't!). On the contrary, their metrics will show them that most endgame players (let alone ones not playing there) play for a month or two and then quit, sometimes coming back after a major .X patch.
I don't think anything will change without major changes to Blizzard leadership again though - the changes that happened after the sex abuse/bullying scandals caused a lot of improvements, but now those people seem to have got stuck in their ways.