We don't really know. A lot of work has gone into trying to minimize incentive for 51% attacks though, and it continues. "Fork off" is a classic argument, that the attacker may take over the chain but then cause a fork where they're left with a worthless system anyway - meaning the 51% attack would only be profitable for a short duration before a fork devalues their winnings and everyone continues on the new system.

Vitalik Buterin is involved in a lot of the theory in handicapping plutocrats, and enabling fair representation/voting in the blockchain space. Check out his blog.