Do miners have both the incentive and ability to keep the reward at 50BTC?
It's not just about the miners, it is also about people using bitcoin. If miners decide to fork the chain and keep block bounty at 50btc, they cannot use those bitcoins in any place where people have not also modified their non-mining bitcoin client. Protocol changes are not just about >50% of miners, but about >50% of user clients. 

In this case, since users do not have any incentive to modify their clients to treat the new fork as legitimate, rather than the original rules fork, any miner that decides to spend his resources mining the new "50 btc forever" fork is just going to be wasting time, since he will not be able to spend his coins anywhere; they'll be rejected as illegitimate by users, merchants, exchanges, etc.