So most Bards and Warlocks, some Sorcerers and Artificers, and all EKs and ATs are terrible and should never have existed?
If your argument is "don't add armor proficiencies after level 1", the playtest Sorcerer didn't do that either, so that's irrelevant.
Having gameplay shifts across a single DAY are just fine--in fact, several classes are built around it.
Sure. Market research which shows that adding armor proficiencies at higher levels is not a great gameplay choice has nothing whatever to do with making classes which change their gameplay over the course of a single day and then revert back at the start of a new day. Completely different things.
But I'm also of the opinion that they should never have done the "subclass at level 3" thing to begin with! Yet another reason why actual novice levels would have been so much better, rather than this "levels 1 and 2 are training wheels" albatross around 5e's neck.
No. Good class design has the same day to day experience from level 1 to level 20. That doesn't mean that the experience from second to second is identical. Much the opposite, actually. The experience SHOULD change across the course of a day for some classes, while remaining almost unchanged for others. That's...literally supposed to be the thing justifying the stupidly high power of magic--it's supposed to run out. (Of course, in practice, I find that it often doesn't actually DO so, even for characters who spend multiple spells in every fight, unless they're Warlocks in typical groups, meaning, those that don't take enough short rests.)