Two days of thinking and I still have no clear answer...
I'm spending a lot of time here, and I've registered at least one game on every server this year. Here is what I think:
1. Servers #1, #3, #6, #8: Short CV goals, quite easy, very popular => we should keep them.
2. Servers #12, #12A, #13, #15: Easy temperate CB goals, very popular => we should keep them.
3. Servers #K2, #31, #33: RoadKing, Cargo, BusyBee - unique in the world of openttd servers available 24/7 + quite popular => we should keep them.
4. Servers #27-#30: APHID CBs - unique in the world of openttd servers available 24/7, carefully thought-out thresholds for requirements; unfortunately: too hard for many players imho, but... => we should keep them.
5. Servers #K1, #K3, #7 - one extremly long + 2 classic hard servers, quite popular => we should keep them.
6. Server #9: Very popular FIRS CV server, one of the best configs here => we should keep it.
7. Servers #10 and #11: Less popular desert CBs, underestimated by players imho, they are quite easy and satisfying, but slow at the beginning => we should keep it.
8. Servers #2, #4 and #5: Long, boring CVs imho -> eventually I could sacrifice them.
9. Server #32: VIP CB Road King -> should be closed.
So, if I think hard, I see 4 servers (out of 25) to close. But it doesn't slim down the infrastructure, so... keep them running, please

And thank you for keeping them running!
Another idea, in opposite direction: replace #32 config to something available to all players, and keep it in MISC category. Now: dreaming of 1M Cargo FIRS server...
--tomb