If this doesn't work for your specific use case, write a wrapper object - your point is that the API was stable with V2, but it will still be with V3 and you should be able to rely on that.
Am I missing something?
The ability to execute arbitrary JavaScript in content scripts at runtime. Remote-browser works by making it very easy to run code in content and background script contexts from a remote client. Typical browser automation tasks like filling out an input, clicking a button, or extracting the HTML of an element are accomplished by marshaling code and results between the browser and the client.
your point is that the API was stable with V2
My point isn't that the API was stable, it's that Google is moving towards removing key features from the API because they consider them to be too powerful. Blocking webRequest and executeScript are two notable examples.
I doubt that tampermonkey executes custom scripts within content script sandbox since a script could take over the extension itself. They should be fine.
Please note that as an extension developer I receive at least once a month a cash offer to inject scripts that track users.