But I wonder at what point digital effects become 'good enough' in some sense that they never look aged beyond the containing film. At some point surely there's no more perceptible 'resolution' to be had.

In practice digital effects haven’t approached being convincing the way practical effects do. In many cases, especially when used liberally, digital effects still clock as amazing digital effects rather than reality. It can be enjoyable but I don’t see what would move forward other than recognizing cgi isnt the best solution for everything.