You’re using a lot of words, but your fundamental premise is off. There’s no such thing* as “simplify the game down to their skill level”.
Any individual piece of content gives you a certain requirement to get it down. It leaves it up to you if you can get there with improving your gear, improving your skill (whether that’s class/spec or general game skill), or employing a specific strategy. Any player can choose to do any of them, or even all of them. In response, harder content doesn’t get “simpler”; if anything, even after you have the gear and skill required, it remains an objectively harder and more complex accomplishment to fulfil (than easier content).
A lower skilled player can improve their skill at any time, just like they can improve their gear. There’s no such thing as being “locked out”, except maybe at the very very top end where someone can look at M+35 and conclude that that may just be mathematically impossible for now.
*Within reason, if people get to the point like with Sarth or certain Mage Towers where you can overpower phases completely and just zerg, that’s content destroyed, but none of those are really intended or designed for.
Aside from what was discussed above, this is sheer nonsense. Harder content and levelling serve very different functions, and no player is “locked out” from doing harder content anyway.
You’re confirming that you’re extremely confused about how gameplay works.
But suppose a player’s skill level is such that they are only good enough for
A person’s meant to end up at the content that’s their natural skill level over time. If you’re a Mythic raider in a guild that normally sees the last boss down, you’ll probably spend the entire patch working your way through the earlier bosses, to then get the last boss down in time for the new patch. In the new patch, you’ll be starting with earlier bosses once more, you are not the person who’s going to get World First and go “I have end boss dead within a month”.
Similar progression works in content like M+. It’s a little more nebulous since seasons don’t directly equate to each other, e.g. in one season +12 ends up at roughly the equivalent difficulty of next season’s +16, or the season after that’s +9.