Executive compensation is often based around share prices, so this can be worth quite a lot to the people making these decisions without any long term upside for the company.

If you want to understand how companies behave you really need to look at things from the perspective of people making the decisions.

I don't see Alphabet share price changing much just because of SpaceX being valued 2T instead of lets say 1T (being extremely generous). In fact this deal will hurt their profits, which is more likely to hurt Alphabet stock price than the valuation of an asset that they hold.