The new API would limit content blockers for Chrome-based browsers and eliminate options to create new and unique content blocking extensions. All that would be left are AdBlock Plus like filtering extensions that would all offer the same blocking functionality.
While there would still be adblockers for Chrome, the limit of 30,000 network filters would make even those less capable than before. EasyList, a very popular blocking list, has 42,000 filters and if users add other lists used for other purposes, e.g. social blocking, that number would increase even more.
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Could this have been Google's plan all along?
Create a web browser and use it to combat the use of content blockers? Block some annoying ads, allow basic content blockers, and block any other form of content blocking to make sure that Google's advertising business improves again?
Some users would certainly move to Firefox if uBlock Origin, uMatrix, and other content blockers would no longer work in Chrome-based browsers. Even if millions would migrate, it would still leave Chrome dominate the entire desktop browser market.
It will also be interesting to see how Opera, Vivaldi, Brave, and other Chromium-based browser developers react to the change, if it passes the way it is proposed right now.