If miners had to be registered and were restricted to a certain amount of hashing power per unique registered user, or had to purchase a standard mining rig (maybe one that doubled as a POS, and restrict mining to merchants) in order to participate, I think that would mitigate the risk of a 51% attack. Right now the imbalance in ability to mine not only creates a risk of an attack, but it skews the distribution of newly generated bitcoin. In my opinion those are the two biggest problems with bitcoin and good enough reasons for it to fail in the long run. It would take a new crypto to address those issues since the initial distribution of bitcoin is so imbalanced.