Your logic on the surface seems to make sense with using a bullpen/throw day and simply using that in the game in relief instead. However, in game pitches, IMO, are so much more stressful than sitting in a bullpen and throwing to a catcher. So now not only is this pitcher not going to be at full strength for his relief appearance but could still also not be 100% for his start. I believe that the short rest thing is a bit overblown to begin with. Sure they might not be as sharp but its typically pretty close. Why not have your best pitcher chew up as many starts as possible as long as he can sustain it. Now, the Bumgarner is an extreme example. It was game 7 and didn’t have to worry about starting again that year so that comparison seems silly than trotting Kershaw out in say game 4 in relief because he would obviously have to go out and start game 6 if necessary. Also, some pitchers recover much faster than others. Case in point Scherzer won’t pitch on short rest.