I'm struggling to understand the point here, I think because I'm not sure of the context this statement is in.It really isn't. It's comparing class/subclass features to class/subclass features.. the things over which a player has agency and visibility.
"Someday you too might be able to do mythic, awesome things, if you can just get access to the right mythic awesome items..and hey you have very little control over the likelihood of that happening" is...
The awesome stuff you need magic items to accomplish now could just be awesome features you have access to as part of your class.
I come back to, some players don't like magic characters, at least not all the time. Sometimes they don't want the character to be inherently magical, but are happy to be lit up with magic items like a christmas tree. And sometimes they want to have no magical gear at all. This is not related to the campaign setting, it's an individual play choice.
If the campaign is a high-magic campaign (and presumably a Session Zero or something occurs where the players & DM collectively decide this?) and the player wants to play a non-magic character concept who ALSO doesn't use much by way of magic items, then they're making some choices about the sort of story they want to tell, and if that's the story they want to tell ... what's the problem ?
If the campaign is a low-magic world and they choose the same sort of char concept, then they're going to be wanting to tell a different type of story. Aren't they ? So, again, what's the problem ?
Bottom line: if they want to be an epic level, amazing, legendary, non-magic martial, they should be able to do that, and it should be a valid playstyle choice. What's missing for you ?