Not big on homebrewing classes & sub-classes. I wish the Warlock was Int instead of Cha, because it would have put another spellcaster using Int and balanced it out a bit. Oddly, despite my heavy dislike of 4E, I really liked the concept of the gish class (Swordblade?), and would love to have seen that as a 1/2 caster class like the paladin and ranger for the wizard.
Your experiences have been vastly different than mine. IME everyone except the wizard, eldritch knight, and arcane trickster will dump Int, with the occasional RP type player who wants to have an average or higher intelligence character. Also, almost every non-bladesinger wizard I've seen dumps Str the same way everyone else dumps Int. This is partially due to stereotypes, but also due to the fact that the "penalties" for a low score in those is irrelvant to the character.
It's a small buff/nerf, yes. This is deliberate. Having a low ability score is supposed to be an actual penalty, not something that is just shrugged off. By your argument, Con is a nerf to every class, since everyone needs it, but no one ever uses it as a primary ability.
If they had balanced the saving throws across all 6 abilities (which I feel is their biggest failure of 5E), this discussion would be irrelevant, since having a low ability score would make you vulnerable to something. Instead, you have 3 abilities (Str, Int, Cha) that can be safely ignored by a large number of characters with minimal drawback.
Using the Encumbrance variant is a great way to prevent Str as a dump score. Dumping Wis is a terrible idea, as you will be surprised frequently and there are a LOT of Wis saves that can be terrible to fail. I'll agree on Cha, and have found only the follower loyalty rule (DMG or XGtE) as a potential penalty. If I could think of something else, I would probably implement it.