Accessibility absolutely does translate to difficulty, be it skill or time investment. And you always have to design for how the end user actually behaves, not how you think that they should behave.

What muddies the waters is that this is a team based game, and people are usually more willing to point out the shortcomings of others than to accept their own. A lot of players feel like others are holding them back, even if that's not actually the case. Even if you use objective metrics of player skill like rating systems, they can struggle to separate out individual performance. And at the end of the day, the only reliable metric is whether you can complete the instance or not.

That's why players tend not to use matchmaking services for harder content that requires more of a time investment. You can't completely automate the selection process, as most disputes arise from a mismatch in expectations and commitment.

I think that exclusivity is a great motivating factor. It's fine for some achievements to depreciate over time, though, because there's always new stuff coming out. Does it really matter that somebody is able to unsync to obtain a Heavensward mount? You probably enjoyed it for a while when you got it and then forgot about it. And there are some things that will always be rare regardless of how far into the future we go, like the Necromancer achievement. If you're concerned about an achievement depreciating, then you need to be selective in what you invest your time in.