@paul @feld MSCI World uses a free float market cap, so spacex will barely change the weights there
Also ETFs *try* to replicate given indexes and they usually have their own rebalance timings, so it will really depend on the provider if and when You'll have IPOed company in Your portfolio
I don't know how it looks on the USA-investor side, but in Europe it's not that bad if one's not invested directly in S&P or NASDAQ indexes