The problem is given the number of Google's services, it can easily "nudge" site owners into implementing it.
They can make it mandatory for adwords or analytics or any other service that they have for webmasters and everyone will have to jump in while it is still technically voluntary.
If your $bank choses to implement it, then it can very well mandate only windows or osx clients. Then you can't use firefox on linux.
This may sound like conspiracy theory but we already have examples of similar restrictions on android apps with safetynet. Bank apps already refuse to run on custom roms. It will not be limited to banks. Someone mentioned that McDonald's app doesn't work unless you pass safetynet.
That is how ridiculous it can become.
Edit: fixed typos