On the contrary, there are MORE viable methods for reuse with a smaller vehicle. They can do mid-air recovery with such an upper stage, for instance, but no hope of that with a Starship-sized upper stage.
I doubt that. SpaceX was going to make Falcon 9 fully reusable, and it wouldn’t have been crippling. Might be too small to launch a full crewed Dragon or 60 Starlinks, but I doubt if its payload would’ve even been halved compared to partial reuse.
But if you throw away second stages, your rate of reuse goes waaay down.
I don't think an 8-ton lift vehicle is going to achieve second stage reuse.