What happens if you have a coin based on "useful work" and you "solve" the problem you were hoping to solve. Now a lot of "miners" leave the network, presumably because they were only there to help solve that particular problem. Now you have a weak system, and a system subject to 51% attacks by adversarial miners, and therefore not a good money/coin.
I don't see how HN commenters (in large part) don't get this. It's the Single-responsibility Principle, just in a different context.
With Gridcoin you can participate in a number of eligible BOINC projects [1] and get rewards for computing on any of them. The network can add or remove projects.
There is a reason why Cloud providers do not yet have "real" HPC offerings (i.e., performance-wise), even though with all the marketing they do to appear otherwise..
https://en.wikipedia.org/wiki/List_of_distributed_computing_...
I'm yet ambivalent on if it's a factor on why it hasn't been able to attract serious attention, but I've heard the argument that doing calculations that have external value outside of PoW breaks the incentive mechanisms.
To me, merge mining on Bitcoin[0] and MEV on Ethereum[1] are evidence against that argument, since we already have those forces in play today. So Gridcoins concept should work.
[0]: https://blog.bitmex.com/the-growth-of-bitcoin-merge-mining/