Rubio could have won. However, he made decisions and actions that hurt his own campaign and ensured he would not win.
Actually, just two:
1. Trying to rely on media, but to be fair, this might've fared better if Trump's strategy of getting free media hadn't canceled it out. Still, ground game was clearly what should've happened.
2. Being so caught up in the old plan as to not really be ready for improvisation.
For 1, He didn't have the money for it. See my OP, it was a calculated trade-off in exchange for more media buys. Walker took the other end of the deal, sacrificed media buys to build up a ground game.
For 2, Again, see my OP, if you read the links you'll see that even local FL politics people were saying that rubio had never attempted any other type of strategy before, and that this was his whole bit.