> This is already a solved problem with Nix shell (except the task runner part).

devenv, a Nix+direnv-based solution, has a pretty cool task runner thing, plus service management.

> I don't understand why there are any other alternatives still being developed.

I love Nix and I believe it's a great choice for many teams, for the same use cases as mise. Nix's paradigm is the future. But Nix's defects are also real and obvious enough if you use it for long. I can understand why someone might see value in trying a fresh new effort, or an approach that asks less commitment.