I am not sure I agree. I dont think this criticism was true with regards to 4e. Even in 5e I still think the main class defining abilities are there by 3rd level.
Yes, but my argument isn't about a class not getting anything interesting in the early and mid levels. It's that so much of the stuff gained at higher levels seem to be the kind of "workhorse mechanics" that you'd really want to have available throughout your career -- not just the very tail end of it.
For example, Foe Slayer is the Ranger capstone that allows you to add your Wisdom modifier to one attack against a favored enemy. Is that really something that's going to have much of a chance to shine in the 1 or 2 sessions that you'll probably get to play with it? Or how about the Assassin Rogue ability of Death Strike letting you do double damage against a surprised creature that fails a Con save? You get that at level 17, and honestly how many chances are you really going to have to stretch your legs with it? That would be a really cool ability to get closer to 10, where you could maybe work in a usage each session (not that surprising enemies is particularly easy), but by 17 it just feels like a missed opportunity.
So yeah, although a lot of the mid and high level abilities aren't core character concepts, they *would* be really fun to actually use more than is possible considering the level you gain them versus the remaining campaign time. Some really aren't even that great for their level, often just duplicating spell effects that the party has had for a long while (Monk's "Empty Body" at level 18).
Or maybe everyone else just has campaigns that last long after reaching level 20?