The funny thing is, I think if Morphy had been born today he would not play chess, or at least quit after two years like he did in the 19th century. Too boring and drawish, no more romance. Romance doesn't win games when your opponent is playing solid.
So the "What if Morphy applied himself to chess like modern GMs do?" Question is even more meaningless than it seems at first glance.
If Morphy were alive today and if he was motivated, he could be a STRONG GRANDMASTER!
He wouldn't make stupid mistakes that elite players today make (Naka Ne2??) and when they went astray he would dole out severe punishment.