All the power that an advertisement network can buy. Especially youtube since it’s owned by google. And advertisers will be happy to have a way of forcing site visitors to run ads/malware or else they will not get served the content.
It’s similar to certain bank apps refusing to function on Android devices with an unlocked bootloader: you want the convenience of an e-banking application (/ad-driven corporate website)? – Your device (/web browser) “security” must be verified by the “authority” who actually owns your operating system, else you won’t. Everyone* will “be loving” their secure devices, because they “just work”.

^{*who is a potential customer buyer and therefore relevant}

Google is trying to use their dominance to actually own the www. The comment/issue section of the github site of the proposal is quite enlightening, if you have the time … especially their reactions on the general dismissal and condemnation of the proposal as unethical.