Why not a Wizard in full plate with a 2h sword casting fireball and sneaky stabbing something with the 2h sword? Because in D&D forever, Wizards are supposed to be lightly armored, if at all, and use magic to boost their defense.
Because in D&D for many settings now the rules of magic require things like intricate somatic movements which armour, and tools/weapons that aren't arcane foci, inhibits. It's not easy to perform the exact motions with your arms and fingers if you're wearing plate. Mechanically speaking it's to allow for the very thing you seem to think doesn't exist: giving each class clearly defined advantages and limitations. If a wizard can wear plate and wield a sword then they get some of the advantages of fighters without any real disadvantage.
On that topic, the way Wizards work in 5e, is also bad. You only get to select a few spells and then you have a certain amount of uses of any of them using spell slots. Isn't that how sorcerers are supposed to work?
It's not "bad" just because it's different from traditional Vancian mechanics. Sorcerers are distinct by a) not preparing spells and b) having metamagic abilities to enhance and augment their spellcasting.
I don't think the changes being requested are bad suggestions. I agree with a lot of them. My biggest change would be use 2e or 3e hybrid.
Whether or not you personally like it 5e is not only the current edition it's also the most popular edition of D&D. 2e hasn't been relevant for decades, you might as well not call your video game a D&D game if you're basing it on 2e.
