The problem with splatbooks is not that they are overpowered, but that they are additive fixes to core. What core really needs are subtractive fixes... taking the broken stuff out.
All you can do with an additive fix is to introduce more overpowered stuff, to balance out the first set of overpowered stuff. But then you are left with a system that is more balanced, but also more broken.
One has to understand that D&D is what it is not because of all the bogus backwards rationales about design considerations that get thrown around (these are pretty much all lies), but because it began as the ultimate exercise in plagiarism. Nearly every single thing that Gygax & co. found in the pages of myth, legend, or fantasy fiction, made its way into the pages of first edition. But all this stuff wasn't made to coexist. How is a Tolkien-style overland quest supposed to happen in a world with Arabian Nights flying carpets and other ubermagic?
Example:
Why is there a wish spell? Because there is a ring of three wishes. And why is there that? Because Arabian Nights. Why is the wish spell ninth level? Because long before epic spells, that was the highest level a spell could be.
So you end up with a situation where Meteor Swarm is a balanced ninth level blasting spell, but it costs the same as the ability to shape reality any way you like.
Because you had to have everything.
There can't *be* a workable "world of every fantasy story". Because in that world, Anisurimbor Kelhus burns Conan into a small pile of ashes on the first day, and then Odin chains him to a rock for eternity in the freezing depths of hell. And in the "universe of every science fiction story", you never get to have your great Federation vs. Galactic Empire clash, because two Culture GCUs* show up and make everyone play nice, whether they want to or not.
You can't just plug different stories into each other, because they all have different notions of the possible.
A long time ago, D&D reached the point where it needed to decide whether it was to be an Arabian Nights universe of superpowered magic, with every player character a caster of some kind, or a Conan-and-Tolkien universe of brave heroes riding around on horseback, shooting bows, etc... perhaps accompanied by the occasional bone-casting, storm-calling, and even fire-throwing wizard, who cannot fly, talk to the gods, create universes, or bend time... because magic has its limits, too.
And D&D refused to make that decision.
* The It's My Ball and You Can't Play, and the Yet Another Gravitas Joke, respectively.