To defend Google a bit, it looks like the change is being put in place so that users would have more privacy – by stopping extensions accessing all active URLs.
In essence they are copying what Apple did with Safari and their content blocking APIs. In this model, content blockers provide the browser with a set of blocking rules and the browser executes them against pages during render & load. Ad blocking can occur and privacy of what the user is viewing is retained.
Sure it's more restrictive and yes will likely break existing adblockers, but it's probably a better model for the future.
There will be arguments that you "can't do what is necessary" to create effective adblockers. That's incorrect.... I've created such an adblocker [1] using the Safari methods and its efficient, effective and high performance – including loading some sites 2x faster.