Sure, but on my WiFi network with my flat mates there would be like five concurrently logged in email accounts constantly connected and receiving traffic from proton. And then behind CGNAT that might extend to a hundred people concurrently logged in with one IP.

The false positives would drive away so many more paying or future paying customers it wouldn’t be worth trying

Only thing I can think of is fingerprinting devices but then that’d be obvious, break their business model and also drive away customers