Yours is the only valid argument. If space datacenters are not competitive with terrestrial (in terms of megatokens/$ or whatever), then they will never work and SpaceX will fail. That's the whole ball game.
But anyone who thinks they can predict those prices in ten years is wildly overconfident.
Space Datacenters eventually get quietly discontinued after finally killing the zombie brand at which point the only reaction is "huh, remember that?"
https://techcrunch.com/2026/08/21/teslas-solar-roof-is-dead-...
Are you saying that if space datacenters fail then SpaceX stock will go up? Then it sounds like you should invest a lot of money in them.
ETA: If you had invested $1,000 in 2016 when Tesla Roof was announced, you'd have $25,000 today. 25x return in 10 years.