The development that slows will not be parts that allow monetisation. They have a rationale. They will be things like security that have no immediate bottom line impact.
It's not like the open source engines won't be worked on. Or other companies who are not ad companies (apple, etc)
Browsers will not stop existing because google cannot make one. At worse google would donate to a non profit that keeps the engine and do the influencing under the rug.
