If we are expecting to level martial characters up to 20... then there will be a transition from one to the other somewhere along the line. The whole reason for having levels is to see a character evolve and get better over time. And in a fantasy world of magic, I see absolutely no reason to keep a mundane martial character mundane over 20 levels. To me that seems pointless.
I equate it to the types of adventures that PCs go on. At a certain point, continuing to do the exact same adventures at level 17 that you were doing at level 3 is pointless and a waste of time. Your levels are epic tier... your adventures and stories should be epic as well. Otherwise it just seems lame. Like back in 4E when someone would write so-called "Epic Level" adventures for characters at like Levels 22-25, but whose plots were merely just protecting a village from some invading monster. Sure the monster was statted to be like a CR 30... but narratively the entire premise and plot was exactly the same things PCs would do back in levels 1-5. So being an Epic level character was meaingless to the adventure, other than changing the monster that showed up. What the heck is the point? Why bother? Just keep playing levels 1-5 if all the stories you want to do are ones that affect characters back in levels 1-5.
Once PCs get up to levels 15, 17, 20 etc... I think they SHOULD start kingdom building and plane-hopping and the like. To give us stories that only make sense when applied to characters that have the experience and reputation and knowledge that comes from being levels 15, 17, 20. And looking at it through that lens, there's no reason in my mind why a Fighter should be stuck as the same mundane swordsman they were 10 levels prior, except that their "numbers are bigger".
Bigger numbers that do nothing other than just being bigger is a waste of time, space, and creativity.