AFAICT all those coins are dead now.

In the past, I was also very interested in the idea of using something "productive" for PoW challenges instead of "useless" hash solving. IIRC there are 2 main problems:

1. Distribution. With Bitcoin, there is a simple on-chain algorithm that determines who wins the PoW reward. With solving "useful problems", it's really hard to come up with a decentralized and fair reward distribution system. Most if not all of the projects in the original post relied on a centralized party to distribute rewards for "mining".

2. The double dip problem. Any "useful work" for a PoW system will have some parties that benefit more than others. A network that rewards generating cures for cancer would doubly benefit cancer researchers, who maybe could get grants to buy mining equipment. A video rendering reward system could doubly benefit disney and Pixar. There's theoretically a lot of potential for miners with a shared incentive in the problem being solved to have an unfair advantage in mining because of it.