Conversely, as long as the people they actually want to target (dissidents, journalists, ...) use non-compromised E2EE it's not very useful for NSA/GCHQ etc to harvest info about all the cat videos everyone else is watching.

It won't help you with those specific cases no, but Chat Control would be the perfect tool to monitor and stop the spread of information between regular citizens who are trying to organize against the government, just look at China.

It's not your cat videos they're interested in. When people are protesting against the government it's vitally important that they're able to get information out as quickly as possible, to as many people as possible. If the government can slow that momentum down then opposition fizzles out. Chat Control would do a great job in service of that goal, it's large scale crowd control, not a targeted attack.

But it makes the people they want to target very easy to spot - just look at who doesn't watch cat videos. The absence of data is data itself.