I said it the last time the survival of the server was mentioned in your post, it is the lack of admin and community curation causing this. You don't have a good admin representation on Payload+ US at least, and based on the responses here, EU lacks as well. It does not hinge on a gimmick of player count.
In the last few months a major event in the US happened and has been an extremely boring and tedious topic on the server that some of the folks LOVE to not shut up about. I come there to play TF2 not to hear folks whine about their views because it's fun to troll certain community members into instigating them into long ramblings. If you believe it's the player count that caused folks to leave, then sorry, you're out of touch. I have actively left early or ignored joining because of certain people being online in the evenings that join purely to stir shit up and I know a lot of other regulars who have as well. The fact that it went up to 40 players was the thing that made me not care about the life of the server anymore because now even the game play sucked.
Now that it's back to 32, we could drill into some of the real issues if we want to discuss how to keep it alive.
If we want to talk gimmicks instead of community curation:
Something I believe may be happening is a sort of "search engine optimization" (SEO) problem. The server browser allows folks to sort by players. If they sort by that, a higher count often appears higher if the server contains that many players. It doesn't sort by total slots, though. So with 40 being a bigger number, it appears higher in a way I imagine many users may be using the server browser. If that's the case, well, sorry -- that's a bad thing to optimize for because it ruins the game most of us are seeking to play as proof by the receipts Puuro provided about the Turbine+EU server (bigger number =/= longer life). Find a different way to defeat the evil server browser/valve being shit at supporting community servers than this gimmick.
I haven't done a search, but what happened to the betting plugin? That was a great feature that caused folks to focus on gameplay, want to stick around, and caused conversation topics that didn't suck. These are the types of gimmicks that should be added that make it worth playing on your servers. Fun cushions around the vanilla-ish TF2 that we all love. These grow people's interest in returning to a fun place to play and talk. I'm a dev and community builder myself, and would be happy to talk or get some designers together to figure things out that could be done if engineer or management power is something you need help with.
Also, please listen to the calls of my EU friends as well or I suspect the issue will cannibalize them away and be even less likely to fill the US server sooner when they move over.