> you still have capital gains from stock trading, etc.

That's the whole point -- without corporate taxation, shares in corporations are more valuable, and the taxation is drawn from capital gains instead -- and capital gains tax should be far higher, of course (and ideally progressive as well, based on your total income).

Sure, companies can "sit on their money", but they already can. And that money is presumably invested anyways, so it's still being productive (on R&D in other companies, for example), and it certainly gets spent eventually.