Godot is open source. That means it's not being maintained by a sole developer - anyone can make a copy of the code; anyone can make edits. The current developer decides what goes in the main version, but anyone can develop their own fork of it - and if the developer abandons the project, or decides to inflict adware in it, someone will grab the last stable version and make a new project where other people can develop that. (Tenacity is a fork of Audacity after its hostile takeover.)
You can, of course, keep using Unity... and hope that next year they don't decide "actually it's 50 cents per install, even for free projects." Once a company has decided to squeeze money out of formerly non-paying participants in their project, they don't stop.