Rokal good post.
Yeah, it's a huge problem that systems aren't really integrated anymore. Like there's no overarching systems philosophy/goal. There's all these various types of content, but they're frequently contradictory in a vacuum and especially so in the context that they're often undermining other pieces of content. It's negative synergy.
For gearing especially, probably the primary source of player power, too much seems to be more or less viable, so nothing feels valuable.
Perhaps one solution would to be to have gear go back to being good only for specific activities (e.g. PvP gear with resilience), but that has a different set of issues. As it stands though, gear is being used as a sort of catch-all reward system and very likely overused beyond an appropriate amount.
Islands and Warfronts are a huge shame because clearly a lot of developer resources went into them, but the final product is not worth that much. Islands are especially bad because the objective being the exact same across all islands every time devolves them into the same boring mess, just with different window dressing.
I'm not entirely sure if this is the right way to go about it, but I think in general Blizzard should look at condensing difficulty settings while also slowly increasing depth (if not also breadth) for class and systems design. The latter is a fine thing to balance if you also want to remain accessible, but remaining stagnant or shrinking depth is not the way to go (neither is change for change's sake) if you want to retain veterans as well. I definitely think the strata of difficulty in content in the game has gotten out of hand though. I'm not completely anti-LFR or anything, but it's introduction was definitely a turning point, certainly for raids.
For raids specifically, I'm not sure where you'd make the prune. Probably the current normal mode? But that assumes nothing else changes and to be honest the whole system might need a larger redesign. I don't know about other people, but I would almost certainly still be very committed if 10-man Mythic was a thing. Obviously that has its own drawbacks, but that's more on Blizzard's end. It makes THEIR job easier only having to balance for one "hard" difficulty. It doesn't necessarily make for the best end-user experience.