Seems such a waste. Launches are expensive. There isn't something useful to put into heliocentric orbit for which the higher risk of a new rocket is worth the tradeoff for a free launch?
Honestly? Probably not... You're also overlooking the very real utility of PR. Launching a car excites people and makes future "useful" missions more likely.
The images of the roadster in space are historic, those images alone could inspire thousands of young people to explore rocket science or physics as a career who perhaps otherwise wouldn't have. That I think is worth more than just another satellite in space or a science project, they also had to test that it could actually lift heavy objects. This was the perfect item to launch to space.
I upvoted you because it's a good point, but consider it was the first launch ever of this rocket so presumably people were reluctant to put important and expensive payloads on it.
Wouldn't that be heavier by volume, have less mechanical resiliency by volume (no internal rigidity), and have more complicated physics (free flowing, capable of convection/rotation)? Maybe a big block of water ice... but honesty, seems to to have pretty limited utility. If long distance space exploration relies on lifting water vs harvesting it, then it's kind of a nonstarter?
Even cheap satellites are tens of millions of dollars. You can't load it up with cube sats like you might for a LEO launch with extra payload capacity because these would have no way to communicate with us - they'd just be floating out there forever.
