I don't think Proxmox is a huge company making lots of money though either... I've used it for free for years. I imagine most people use it for free. I'm sure there are companies paying for it, but I don't think it's some monster money maker?
I bet that they grew a lot with VMWare/Broadcom changes, even the somewhat paid subscriptions. But they are not yet one of the (commercial/enterprise) big names. I suppose that this donation is part of their "now that I have a positive budget, what I do with it?" plan, and finance growing up both as company/support as in adding features.
