I am not fundamentally disagreeing with you here, but that may have something to do with how magic and spell options often serve as the greatest customization options for classes, particularly when comparing a subclass that has access to spells and one who does not. Spells provide a lot of options. In some respects, maneuevers act as a spell-like system for martial classes from the heydays of 4E.
I don't disagree here and recognize spellcaster vs. not spellcaster is probably one of the major ways of differentiation in the game... But then I wasn't the one who claimed an EK wasn't differentiated from a BM (which in and of itself without some type of explanation besides blatant dismissal makes me even less willing to accept the premise)... I'm arguing the opposite and asking for an example of two subclasses that aren't really differentiated. The issue seems to be this was never a thread to discuss the actual assertion but instead one where the assumption is already assumed to be true.
I'll look at another 2 subclasses up to level 13 that are non-magical... Thief vs. Assassin
The thief can disarm traps, use sleight of hand, open a lock or interact with an object at a much greater speed than other Rogue subclasses.
The thief becomes an expert climber... climbing costs no extra movement and his Dex increases his running jumps
The thief gains advantage on stealth checks when only using half movement
The thief can use any magic item
The assasin is proficient in the disguise kit and poisoners kit
The assasin gains advantage on attacks vs. creatures who haven't taken a turn yet and attacks with surprise are auto-crits
The assasin can, using time and money, create disguises and identities that do not fail
The assasin can unerringly mimic the speech wirting and behavior of another person
I'm not seeing how these two aren't differentiated, one is clearly better at avoiding traps, breaking into places, using the strange artifacts he may find and getting into or away from a heist... and the other deals in murder by surprise, catching victims unaware, and social deception. I guess what I am saying is I can understand if you want more subclass abilities, that's a preference thing (like wanting more classes, or monsters, or feats) and not a "problem" with the game... but the stance that the subclasses don't server to differentiate and it is an actual problem with the game doesn't seem, at least IMO, to be supportable or true..