>People can still pursue their hobbies and sharpen their programming skills by working on more productive issues that exist.

No, they can't. People pursue things that interest them, if it isn't interesting they wouldn't do it at all.

This is similar to "why donate to cause X when cause Y is much more important?" well if cause X didn't exist, those people likely wouldn't donate at all, so it's not really an opportunity cost.

Except I'm not saying there is only X and Y. I'm saying there is AAAAA-ZZZZZ and people working on problem XYZAB would be able to help actually push these operating systems forward if they chose another problem they also found interesting. I do not believe that it is only possible for these people to be interested in a single problem. I think these people can be motivated to find another problem more interesting.