Thankfully if you bought it already you still get to reinstall it, and keys can still be found on sites that resell, which as far as we know they will have to honour.

I have always promoted GOG as first choice for the same reason. You get to keep a local installer if you want it.

Some theories have been going round, and it seems very possible that their lack of willing or ability to refactor those old games to work with their current network is why they chose to switch off the support.
UT3 is to be put back out once they move it to their current system.

Some of the old Unreal games don't need a centralised server system because they are able to use multiple masterservers, which can be changed in the config file.
Up to date Unreal and UT99 users are commonly connecting to 5 or more masterservers, so they will only notice the lack of servers that only uplinked to Epic.

There are several options for replacement of the old GameSpy network because of the broader implications when GS went offline.
A few companies and communities adopted 333networks as their replacement, and have so far had a reliable service with more uptime than before.
OpenSpy has also taken great strides in adding support for many abandoned games and is another drop-in replacement for the old GS networks.
(both are open source projects).

For many games that are hard-coded, or even for console games if you use a drop-in replacement (meaning you don't use an extra client), all you need to do is edit HOSTS or your router to redirect from the old domain used by your game to the IP address of the replacement.

If Epic have truly abandoned these games then maybe they will get community re-releases with all the modern fixes and updates.
Duke Nukem has never seen so much love as this year.