This is perhaps the biggest risk to Bitcoin's future. Does anyone have a convincing argument why this won't happen, or proposals on how to combat it?
Don't fix it, Maybe in 40 years time (with a little blockchain pruning) the actual computation of transaction hashes will be rather trivial and people will run full nodes and mining software on there hand held devices? maybe the small amount of resources needed make it possible to build mining back into the clients, maybe because of the triviallity of required resources, anybody that has interests in bitcoin will run these implementations to keep the network secure even if not for mining, people are altruistic, look at the Fold@home gene mapping cancer research that people run on their computers.
OR
Maybe competing crypto-currencies will be on par with Bitcoins market share, maybe we will have multiple local cryptocurrencies and if Bitcoin dies people will just move, 40 years is a very long time in computing & technology.