For me the first question when making a class is: Does it have a unique mechanical and thematic niche?
The second one is: Would making it a subclass take away the things which make people want to play this class?
Sorcerer for example has a unique thematic niche, but has struggled to carve out a mechanical one. Its mechanic was spontaneous casting, which now all casters get. Now it's just the metamagic feat turned into a class. As such it's one of the classes people suggest axing the most.
Warlord has both a clear thematic and unfilled mechanical niche, as such it's absurd that it's not a class.
Psion has a clear thematic niche, but with psionic powers all turned into spells, and a spell points casting option, it's mechanical niche has been heavily eaten into.
Swordmage is the most troubled of the lot. It's theme is muddy, just being something of an 'arcane version' of a paladin or ranger. Its mechanics have also heavily been eaten into with both the paladin and ranger now having a bunch of magic abilities which they can utilise via weapon strikes. The reaction based teleporting isn't really touched on much this edition though.
But the swordmage also has the issue of not really fitting into a subclass. If you pick eldritch knight or bladesinger you lose the entire spellstrike thing which is open to paladin and ranger. If you pick paladin or ranger all your spells, theme, and class abilities are wrong. Or you have to have an oath. You pick bladelock you're forced into swearing your soul to a patron. If you pick battlesmith you end up with Preston following you everywhere.