> 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.
What's the trouble? What compells you to "bring the fork up-to-date"?
> The fork might not have new features, security updates, compatibility upgrades that I want.
I really don't understand what point you think you're making. The scenario you're discussing is either a) contribute a patch to an existing project, or b) write your own from scratch. Does scenario b) magically introduces feature and maintenance work on your alternative implementation?
> 2. I think it is in bad taste to submit PR without intention of getting it merged (...)
Why are you even considering this a scenario?