Just to clarify: Any company which got open source model is awesome.

Just in that case transition may cause that they core product will not be evolving as fast as we would wish. Without digging depth, things like "Advanced load balancing" just in paid plan makes me anxious.

I can't speak for Nginx, but it seems to me their enterprise product is very reliant on their open source product. Their customers will probably users of the opensource product that seek a little extra.

If they would slip up and let another open source software be better than their open source product it would eat in their customer base and eventually cost them money.

This is why this business model is so awesome. It is both excellent for the users, who benefit from having the core contributors be paid and highly motivated to improve and maintain the project, and good for the contributors who are finally able to work on the project they love without the distraction of having to do other work for money.

That's not even mentioning side effects like the fact that paying customers are more likely to give feedback and report bugs/missing features.