Someone will most likely create a fork to remove this or an option to disable it will probably be baked into about:config.
But that’s the issue - if WEI passes, EVERY webpage will be able to use DRM. So, just like you have to switch to Chromium for DRMed media content, you will now have to switch for every website that has decided to implement it. So, your bank (because google is pushing it as a security feature), Youtube, Gmail… Just like you are not able to play DRM media, you won’t be able to visit DRMed websites without WEI API supported. It’s not something you turn off.