Don't forget where your paying customers came from.

They didn't find your closed source plugins, they were funnelled in from the open source project. The split license plugin model works because both sides of it are useful to different groups of people.

You get to upsell your free users additional features or support.

But more than any of that, all users complain, even paying ones. Especially paying ones. Don't kid yourself into thinking going fully closed will stop that, the difference is you'll just be dealing with fewer people. That's not as good as it sounds.