I think that you pretty much answered yourself about glacius, despite any auto-download "issues" (rather excuses as note that just to start with all those maps are already
included on the provided official UnFreeZe package a large group of people is either currently using or at least have downloaded/installed at some point) the
real root cause is that the
majority "haven't (bothered to) play(ed) other maps since 1999" and they aren't going to start just now that they are adults and don't even have time because some guys at the forum/server feel like it. that's the reality, there's really no significant amount of players with the maps already installed and with at least plus hundred hours of competitive online gaming experience that place them in a competitive position on every single one of those maps as we speak so they clearly aren't going to risk their elitist reputations and wasting their time joining a new map to start figuring out where the items are. people really like doing those things they are "masters" (or at least skillful) at, there's little "fun"/reward for someone who spent plus five years of his live "mastering" (learning) the standard maps to now go and throw away all that just to join a custom maps server to get
raped like a noob and have to go through all those plus five years of dedicated hard work and suffering just to be able to enjoy a game again. beside "mastering", or at least be competitive on, the custom maps typically require a level of skill considerably higher than that of the standard maps and that it's something that generally far exceeds the abilities of the average public server gamer. the reality here is that customs maps are assets created by elitist players mostly to meet the demands of other elitist players and per definition the elitist portion of a group of people is always a minority. in the other hand the standard maps are assets created by a commercial studio for the purpose of being commercially successful and to accomplish that they obviously target the typical regular player rather than just a reduced elitist portion of them. for the most part that's the untold truth here and that's why
exactly as I foresaw outside of the organized meetings here and there, that are obviously not a sustainable playing model for a
public server in the long run, glacius peaked at five or six two min connections a day at best with most days doing zero connections while spacius just got games going and gamers involved right away despite obviously falling short from glacius (according to gameplay standards).
for the rest, yes, of course, I know the space maps are obviously rail dominant maps and they overall fall very short behind the standard of gaming quality that is offered at main UnFreeZe. sure, that's exactly why those maps aren't on the main servers, just as I said. that being said, I insist, it doesn't mean they are just unplayable either. I think people can (and do) also enjoy playing them and serve the purpose for a third supporting server as well as overall bring some variety to the community just right.
as for using the server idle cycle to serve download frames instead of just sleeping for the remaining of the frame time or until a packet arrives, I already know about that, no big deal, the code you talking about is the one on this commit
here. but just as I broke and I explained several times, no sane person should ever allow in-game downloads according to 2020 security standards and I won't even serve any downloads in-game either. not to mention the game comes with these downloads off by default and for a good reason, they are potentially a great way to distribute a virus or a malware. but ok, let's even forget that for a moment and assume that the auto-dowloads would always be legit, there's even no way to tell that the files you're serving aren't going to conflict with other ones the client have installed. here a very large group of people (virtually the majority) is playing all with different files and most often than not when you start just dropping files on your installation like that you end up with conflicts and all of the sudden you end up with broken textures that fuck up your maps, broken shaders that fuck up your models, broken files that just fuck up your weapons or your game just start crashing out of nowhere (some maps for example don't even work on all engines/cfgs) and when that happens then you have to go through a ton of files to figure out which one was that your fav smart ass admin placed there that fucked up your entire game, no good, trust me. so look, there's no way I'm gonna go through all that trouble just to get a couple of noobs to disconnect from the server after a couple of minutes of loading a custom map at best.
that being said I don't discard I'd ever get back glacius again provided there's really enough interest but that would be it. I'd brought it back as-is for some meetings and so but I won't fuck with the well know established servers at all nor with any other kind of downloads outside of this site.
Something different. Where strategy and teamwork isn't as critical as there is not enough time to have one before the round ends with -3 frags 
... but it let's some steam off and just generally have a few fast rounds. Honestly, playing on spacious made me laugh a lot and was super fun! ...it's a different dynamic. I wouldn't expect anyone to be "the best" on these maps, who cares. It's just good fun, and it's amazing to play unfreeze on these classic maps. I really enjoyed the games there so far 
Actually map selection on fpsclasico servers is pretty wide. I still haven't achieved my goal of recording a single game on each map, on my youtube channel.