I always liked many of these maps, however players start to complain after a while about one map or another, then decide they won't play a particular map, so swap server. This ends up with other players leaving the server to be with friends, or you get some wild accusations flying around that admins do not listen to players because maps are either removed or not removed. Other maps which played at an acceptable level at one time, become sluggish and have bugs appear, due to the changes made with the Valve coding. Some of the older maps have texture problems, due to the type of textures used or the map is made in a certain way at the beginning of its life and Valve make more changes and the map becomes unplayable. A prime example of this would be de_rush, a popular map, but with pretty bad texture problems and increasing lag for many players.
As an example, I always enjoyed back alley because with a concerted rush tactic, the map goals could be achieved, however many players did not rush and just camped every round, so you ended up with a stalemate. However either due to changes in the code or the way the map was made, this became increasingly laggy at one time, so was removed.
VIP and other mods like Zombie have been suggested in the past, but these of course should be on dedicated servers. The thing about any additional mod, is that it must integrate with the UKCS mod, that everyone on the servers have come to expect, so is not a simple matter of throwing on a server, adding a predefined mod and sitting back on watching it take off. Servers cost money and time not only to set up, but to administer.
Surf maps have been suggested as well, and some of these have been tried, however the reaction from players is not favourable at all. Server #4 now hosts an assortment of maps that used to be on the main servers, but again removed because players complained about the type of map.
UKCS has to look at what the majority players want on the servers, but tbh players can change their minds from week to week, whilst others oppose every single change.
One idea would be to run a regular rotation if you will on a particular night, with the older maps on.