Thing is Google lost money for many years in YouTube. Nowadays I think it's profitable but it seems unfair to ask a company to take loses for a decade or more and then force to sell it when it's making profit. If we set that precedent nobody will take risks with the next YouTube like company that loses money initially.
So you believe that companies ought to get immunity from antitrust regulation simply because they made investments in the hopes that they'd be able to profit from their ability to dominate the competition?
Regardless, if the shareholders receiving stock in the a spun off company, so is not like their investment disappears. No one (should) care about some personified "Google" as if a particular corporate structure that happened to exist was actually a human being.
Also, Youtube prints an absurd amount of money, it isn't like this is some sort of change that is happening just at the moment that it finally making some money.
