"90% of everything is crap." -- Sturgeon's law [0]
The problem is that once someone has put in the time to create a PR, declining it is never going to feel good for the contributor.
Yes, they can maintain their own fork with their changes. But to be told "this doesn't work for our project," no matter how politely, is often emotionally interpreted as a form of rejection.
Which is what spawns the kind of rant above in the original link.
I would also agree you are going to get some amount of crap no matter what. But, i would say if you want this to be different, it's not entirely on other people. Even with the best of intent, you can't expect contributors or teams or processes to behave magically just because they should. You usually need to motivate/incentivize or train the behaviors you want, and iterate the processes and your approach until you are getting what you want.