No, it isn't.
Open source developers are not under an obligation to continue providing their time for free. If they choose to pivot the project to some form of monetization, that's their right.
Previous versions remain available under previous licenses - that gift to the world remains, and stays available for communities to fork from, should they choose to.
If a project gains sufficient traction and delivers sufficient value that people depend on it, it's in everyone's best interest that the developer finds a way to monetize, so they're motivated to continue working on the project.
As you say, then the market will decide if it's a price worth paying, if they can live without the service, or if they'd rather take on the build burden themselves via a fork.
But Free Today does not entitle you to Free Forever.
