I had similar thought and I think that doesn't work well in practice:

1. If I have to get back to this project several months in the future, it might be a lot of trouble to bring the fork up-to-date. The fork might not have new features, security updates, compatibility upgrades that I want. The upstream may have rewritten the code, making my fork redundant or refactored it and I have to rework it again

2. I think it is in bad taste to submit PR without intention of getting it merged - it create open ticket that maintainers could perhaps never close and have to sort through.