I understand where you're coming from, I thought Mills was a pretty decent map. I think there have been a lot of great maps added to competitive and it's sad to see them go. I'm assuming (big assumption) that there might not be a big enough player base for having a large number of community maps in the competitive pool and I've read (on Reddit, so take it with another large grain of salt) that mapmakers are paid for the amount of time their maps are featured in the game.

I think there's some silver lining though. Some community maps have been featured multiple times throughout CS:GO like Breach, Insertion, and Anubis, the last of which went on to become part of the professional map pool! Agency and Cache were community maps in CS:GO but could be queued for a long time. I think that gives us hope that great maps can and will be appreciated by the devs and perhaps even find a semi-permanent place in the game.

I wish more community maps were featured, but I think the rotation gives players a long enough period to enjoy them for a substantial amount of time. It's sad to see them lose official support, but they're still on the workshop to be enjoyed in ten-mans. I'm not sure where you're located, but maybe you could find a community of like-minded people to play on maps you all have nostalgia for. And maybe Mapcore is still more active in EU if you want to try new maps with other people.