There are only two solutions to this problem:
.1 Switching to tail-emission and forgetting about "there will only be 21m" mantra.
Peter Todd has actually been entertaining that idea of adding a tail-emission or extending Bitcoin's inflationary period indefinitely. But it's going to be very hard to convince the community, and many people believe that the network has ossified to the 21,000,000 total coin supply. But perhaps if the situation will be life or death for Bitcoin, the community might start to entertain a discussion of a tail-emission.
.2 Switching to PoS (which will make it even more centralized)
POS doesn't work.