If that happens I suspect they would lose a large portion of their userbase to Firefox. The main selling point for a lot of people is that these browsers are basically Chrome without the Google stuff. If they change to Quantum, why would people not just use Firefox? Unlike Chrome, Firefox is already fairly privacy focused, there's not as much of an incentive for people to use alternative Firefox-based browsers.

I currently use Brave, but if they switched to using Quantum I personally would just switch to Firefox proper.