Counter-argument one: things known by Google represents another dose of data into a single place which accumulates way too much of it already.
If I watch something on a peertube, many may know that my IP streamed that, but they don’t know who I am because they’re not an all knowing internet-scale privacy-violator. Best of all, google won’t know I watched it at all, so it won’t be another annoying data-point in the super-aggressive youtube bubble.
Why do you think that? It would be very easy for Google to observe who is viewing PeerTube videos and to link that back to those people's YouTube profiles. You may trust them not to, but if you do, it seems you're half way to thinking it's OK for them to know.
I'm not saying it can't be done, but I wouldn't assume they would go out of their way to monitor this, just like I wouldn't assume they're monitoring public IPs of torrents in the process of downloading.
Yes. It would be easy. But why would they?
Speaking in BT-terms... Peertube's might find this peeking annoying and start publishing "peerguardian" like lists to prevent Google-spying.