I think there’s really not much coins that are mathematically innovative that are considered part of the mainstream coins or those with lots of support and value. There is one though MATH.
It also involves prime factorization among other mathematical computations. Its price is 0.5 usd so it’s not as bad as I would say. There are a lot more out there that aren’ just explored or probably have been long dead already
Of course not. Nowadays, there are many "shitcoins" that are straight clones from the other. Particularly "meme" coins. Developers shifted their focus from quality into quantity. I'd say Primecoin is a kind-of-a-kind PoW cryptocurrency that made its mark in the crypto industry. The code is open source, so what's stopping someone from improving it or making a new project from scratch?
It's a pity that a good project like Primecoin was left behind in the dust. Peercoin was made by the same creator but it's still actively maintained by the community. I guess there's not much interest in PoW coins these days. Hopeefully, there are other alternatives meant to keep the idea of prime number mining alive. Who knows what surprises we'll find in the future?