Inflate bitcoin, e.g. remove the hard cap and allow for greater block rewards to sufficiently incentivize mining
This is bound to happen sooner or later, but there are still many variables with this option.
Personally I would be in favor of this:
increase max block size to 2Mb in the near future, and increase the limit with 50% every 52596 blocks (≈about 1 year). But any of the proposed solutions currently being discussed is WAY better than ongoing debate and no solution at all.