In that case you could maintain a set of patches for upstream, instead of going for a full fork. You could document how much time you spend maintaining those patches and communicate that to your manager. If it is still cheaper to maintain that set of patches and not pay upstream, I guess that is a business decision that might financially work out.
"paying upstream" is fine for little independent open-source projects, but for the big popular dependencies it doesn't actually work. You can't just pay Google to work on Android/Chrome, or pay Facebook to make changes to React...
Using software from Google or Facebook, it should have been known when making that decision that there is no influence you can have on those. These companies are run by psychopaths.
I do understand your point though, there will be many other software projects where the same issue is happening. I guess maintaining your own set of patches or doing a full fork are then the options. Or refactoring.
