I think about this a lot... what will happen to the network once close to 21m BTC have been mined. There is no reason to carry on mining and supporting the network.
I think in 127 years, processing power wont be an issue, just like ASIC are emerging now, new technologies will keep us up to speed so that it wont be a concern. Plus the transaction fee can be increased per transaction so there ill always be a way to make it worth someone's HR to do it.