They didn't leave enough levers to pull in their quest for streamlining...
Why do you hate fun?
Joking aside, I still don't get how a subclass could give you trouble thematically. Most of them are pretty in line with the rest of the class and don't need anything huge to justify them. At all.
Also the PHB has plenty of jankiness. Backgrounds barely mean anything unless your DM is really into them, Feats are too few so if they impact a build it's almost jarring... for the Fighter a lot of these are basically class feature the Fighter was forced to share with the whole class (Seriously, if Polearm Master, Heavy Armor Mastery, Great Weapon Master and Sharpshooter had been lv 6 'Advanced Fighting Style' nobody would have bat an eye but I bet someone would have been all "WhAt AbOut eXtrA FEATS! The FigHtEr NeEds eXtRa fEAts!!" so they did that). And then the Cleric and Bard only have ONE attack cantrip! Great for diversity >< and let's not talk about the Ranger... Is the Champion even that popular aside from Multi-class shenanigans? MC-ing is not the end-all, be-all of customization because it's actually easy to mess it up with subpar builds that don't DO anything.
I would also argue the PHB Gish options still don't feel like proper fusion of Magic and Swordplay. They're just a guy with a sword who SOMETIMES uses spell with a free hand. SCAG cantrips were a much stronger foundation for a proper Gish.
So for some classes we really needed the extra touch of additional subclass. For others, sure they could have lasted longer without extra (why did the Wizard get EIGHT of 'em?!) but then it wouldn't feel that fair.
Plus, as I said... FUN!