I think its a legitimately weird thing specific to software where people release something interesting as open source and free, it gets used, they feel an obligation to make building it their job, and then they feel anger when they don't make money from it. I understand the feeling of being slighted when there are billion dollar corporations using your software, but it was also a situation you opted in-to in the first place, with no expectation of reciprocity until now.
I support the idea of asking people to pay for your time. That's totally fair. I think its also a little icky to say "pay me six figures or fork it"; so, is there no intention to actually make this an open source project? One where people contribute code instead of money and its supported by its users in that way? This project has over 100 contributors.