I am no expert, but I wouldn’t be surprised if full reuseability of the upper stage turns out to be a technological dead end.
It seems to only start making remotely sense on really big rockets and once to got all that mass into space, why not keep it up there to do something better with it?
All depends on turnaround time and cost. It might be worth the payload hit if you can simply refuel the upper stage and go again the next week. But it’s a hard, hard problem - reentry is quite spicy for big vehicles like that.
Look up Stoke Space. Their concept is like having a giant capsule as a second stage. They still have a lot of work to do, but it’s very different from Starship and has had some promising engine tests so far.
Well ideally you’d only be trying to re-use the upper stage portions that serve their purpose in a launch. The payload should definitely stay in orbit or beyond.
