Not an expert at all but from my understanding a traffic correlation attack doesn't require someone to run the relay he just needs to see what traffic enters and leaves it. So the German BND for example can just go to Hetzner (15% Tor traffic) and ask them to mirror the traffic of all relays to them. They don't have to run any relays themselves.

Alt227 has a point but the Tor network is centered around a handful countries where traffic is cheap and there aren't that many huge IXs and Tier 1 ISPs where much of the traffic flows through.

I'm not saying that this is done but it's IMHO more likely than state actors running thousands of relays.