Even if other things can't be done, just finding prime numbers isn't exactly useless. Maybe there are similar things that can be found. Bitcoin can be altered by consensus--a necessary survival feature--so it's not as though this fact is really a knock against the continued use of Bitcoin. When you need to adjust the difficulty, just give multiple problems. Primecoin manages, but primecoin is silly because Bitcoin could obsolete it easily if it chose. We could even choose which kind of problem we want to solve (if the network scales difficulty to be the same) when we mine, if other alternatives exist.
I think people are overrreacting to the OP as a knee-jerk reaction to defend Bitcoin, when really that's not necessary. Bitcoin is the best option for a money system, but it's not perfect and can always improve.
In my opinion, if you can secure Bitcoin, and do something useful, like finding prime numbers, but maybe more useful
Perhaps a proof of work system could be established in a future coin, based off of protein folding.