The sustainability of open source doesn't seem to come down to whether the product is open source or not.
As an example, Elastic was a $10B company in 2021 — the year they went dual license — with revenue of around $500-600m. They showed that you can build a huge business on something that's given away for free. I don't know anything about Synadia's financials. It's possible they're successful, but simply want more. It's possible they've not been able to build a sustainable business on NATS. Their commercial offering is basically support plus a closed-source control plane, which isn't exactly a big carrot when the alternative is $0.
I also wonder if there's any VC pressure happening here that could explain the sudden shift. Synadia raised $25m in 2024, and it may be that, one year later, the investors just aren't seeing the progress they were expecting.