I think the rules are there are hard limits unless a multi-trillion dollar company IPOs with a significant absolute float, in which case tracking the "market" obviously includes said company.

Yeah, this is correct. There are so many large multi-trillion dollar companies coming to IPO, which if your are passive index holder and you are trying to track the market it is correct for these companies to be included. And besides SPY has chosen not to fast track where QQQ has. It is a free market, and folks are free to NOT buy QQQ. So I'm not sure why this is a point of debate.