This is the danger of having a single dominant crypto-currency. By having hundreds or thousands of options, this becomes impractical. Some, like Ravencoin do not have the concentration of mining into five large hands. Instead, the mining is distributed more globally and more disbursed -- truly decentralized.

I'd also like to point out that 51% is not enough to censor transactions unless the 51% is so fully coordinated as to not accept and build on the nearly-half of the blocks solved by the 49%. The game theoretics puts each entity at a mathematical disadvantage unless there is 100% coordination. Effectively the prisoners dilemma.