Even at the far slower speed the first stage would still be damaged from re-entry. That's why they do a re-entry burn to slow the stage down before the atmosphe

Even at the far slower speed the first stage would still be damaged from re-entry. That's why they do a re-entry burn to slow the stage down before the atmosphere gets too dense. There's apparently also some interesting physics on the engine firing dissipating the bow shock which helps keep temperatures down.