I have no idea if SpaceX will ever make the upper stage fully reusable. The space shuttle having existed isn't an existence proof, given the cost of repairs needed between missions.

However, with Starship SpaceX has both done more and less than putting a banana in orbit. Less, because it's never once been a true orbit; more, because these are learn-by-doing tests, all the reporting seems to be in agreement that it could already deliver useful mass to orbit if they wanted it to.

But without actually solving full reusability for the upper stage, this doesn't really have legs. Starship is cheap enough to build they can waste loads of them for this kind of testing, but not cheap enough for plans such as these to make sense if they're disposable.