If it wasn't considered a solved problem, we wouldn't have public CA's for our browsers to trust, as their very premise is based on having a secure way to store private keys. I think a FIPS-140-2 level 4 HSM and hashicorp vault enterprise will cover 99.9999% of all other use cases for private CA's and PKI.
Security is solved in these use cases, but not usability while maintaining security for personal use.
Keychains, Vault, GPG, HSMs, password managers. It's solved, just requires effort and understanding to setup. It's neither "huge" nor "unsolved" as pks have to live somewhere and cannot exist in the aether.
Yeah, I meant that the security tradeoff with usability is unsolved in personal use. It requires too much effort and understanding. If you use hardware keys, you still have to backup your keys somewhere safely.