> "Everyone" includes other users who would benefit from the fix,
They can still benefit from it by getting the fixed version from the PR author.
> the PR author who can continue using the same version as everyone else,
In other words, the PR author is expecting the maintainer to do the work of upstreaming and supporting their (the PR author's) fix for free.
> and yes, the maintainer who is foregoing even the possibility of the fix without starting from scratch.
And that is their prerogative. If they actually really want that fix they can always get it later. But as said by OP, chances are really good that the way the fix was done would conflict with the direction they want to take the project. And that would actually be a net loss for the maintainer.
> As I said in my comment, it makes total sense to me to look at a major change or a risky fix and say "no this isn't worth my time". But I'm sure we've all had those bugs where they take forever to track down until eventually you find a simple problem in code that is obviously an error. To not even take a look at it, as a rigid matter of policy? Yes, I absolutely think open-source maintainers would be missing out on saving some larger efforts.
This basically boils down to, expecting an open source maintainer to do the work--for free--of maintaining other people's patches in a patchwork codebase that they no longer have total control over.