As an open source project no one is forcing you to maintain it. Every fix you put in is something that you do of your own volition. No company can force you to accept a PR or work on it. I think FOSS developers often get stressed about this but unless you personally have financial motivations around what you’ve written you can tell people to fuck off. Yeah they can complain, but you have zero obligation to fix.
The sponsorship seems to introduce a business model around what is FOSS, then it’s not FOSS anymore. The entire purpose of FOSS is anybody can copy and repurpose what you’ve built. They can fork it, take it in a different direction and create a business off of it. Depending on the license you’ve explicitly agreed to that.
This sentiment is going to be unpopular but I think the outrage is unwarranted.