| This is a frequent point that comes up from people who haven't really thought through all the details of how PoW based cryptocurrencies work. A good proof of work function needs two things (among other desirable properties I will elide): 1) For some difficulty factor D, you should be able to generate an instance of the problem that takes time proportional to D to solve 2) The solution should be verifiable in time much less than D (preferably constant time) So until you can show me a foolproof way to generate protein folding problems, genetics problems, or SETI problems, etc. that have these properties (I haven't found one myself), it seems quite difficult to make a "useful" PoW. Now, theoretically, if you found a "useful" problem that had property 1, but not property 2, you could convert it into a viable PoW using efficient zero knowledge proof techniques [0], but at the moment that isn't really viable. [0] - https://eprint.iacr.org/2013/507.pdf |
On the plus side, the problem does not necessarily be "proportional to D", in fact, finding a better function would be incentivized. Since the computer cycles would produce a "real" value (curing cancer), the upper limit for these coins would be the amount of money society would spend on a cancer treatment.
/speculation