Well that's obvious, but how exactly?
At the very same time as we've got this problem, we've practically got an army of people screaming at the top of their lungs that group finders and things like that are the best thing ever, precisely because it allows everyone to see all the content with little to no effort early on. In a world where a significant number of players want that, you cannot deliver compelling progression.
Meaningful progression would be that the bigger numbers let you do things, see things, and complete things you could not do previously, but if all the content needs to exist in a version designed for literally everyone regardless of whether they have done any progression at all, then this cannot be made.
Preach's argument is completely on point. What exactly is he preparing for in the new tier, again? He farms gear only to have it all replaced almost instantly, and even if he didn't get it all replaced instantly, he could still see all the content with all of his friends in a difficulty lower. There is literally no incentive for him whatsoever to farm gear and improve his character.
But woe be you if you try to explain that this is a real and genuine problem. You'll get called an elitist, you'll get flamed, you'll get told off by by legions of people who are holier than thou and who believe that, because they pay a subscription fee, therefore they should have access to everything. If you try to bring up the fact that you don't even want the majority of content out of people's reach - heck even mage towers and maybe a couple of raid bosses per expansion would do - they just ignore you and keep going until you've been driven completely insane and just give up. Preach has been there, too. Remember the video "The Problem In the Mists"? This is just an extension of that. Blizzard managed to defuse the problem slightly with set bonuses (and then not dropping them in LFR and M+), but in their absence we're back exactly where we started.
So what's the solution, then? Oh, timegating! If we can't gate things behind skill or farming or gear or any sort of personal effort, then we must gate it behind the sheer passage of time! It's the only option. Except then we get ... well, timegating! It's just not fun or engaging and people are justifiably annoyed that the effort they put in doesn't mean anything. They just need to wait - there is no incentive to do anything else, and that's where Preach has found himself now.
It has to change. It needs to change. If it doesn't change, WoW won't have long left.