But how are we going to emulate proprietary online services for games relying on them?
Games preservation should be legally enshrined, and require client and server source code to be published if a provider decides to stop running the online services required to play.
This is a fairly lofty and unrealistic goal. Unfortunately, the right for companies to keep their source code private isn't going to go away anytime soon and if they were legally compelled to release binaries, the setup for a modern cloud based online experience is not for the faint of heart.
A more realistic goal would be to say that all products should be usable offline (with exceptions for impossibilities like an instant messenger or something)
If the online servers don't exist anymore, there should be a path to functionality without them. For everything, given the rise of iot especially. If there's a path to functionality without the online service there's a path to preserving the game
Private servers don't really happen much or at all anymore, "here's a .exe you can run" idea doesn't scale on modern online infrastructure well
Emulation is typically a very difficult thing to do, often requiring cracking the original game to get it to work with non official servers and also mapping and building out all the online subsystems. It's rare.
Well, if your games is popular enough some may start to do revival project or create these custom servers.
Back in late 2000s I rememver my brother who used to play WoW on private server (which unaffiliated with Blizzard) and mostly these unofficial server are popular for MMOs game back then.
Nowaday, you can have something like OpenSpy which emulates GameSpy servers runs by communities. It is all depend how deeper you want to venture each games.
What you can't preserve is the joy of playing on period correct experience :)
Not that most modern multiplayer games are worth preserving due to their toxic design, but this isn't a huge issue. BF2 servers started back up thanks to Russians loving the shit out of that game. Warcraft 3 is still very much playable online and NOT on battlenet thanks to W3Connect. Fightcade made 90s 2D fighters playable online. Numerous console emulators support netplay.
There are lots of examples of online services being REd to bring old games back to life, but doing it after the service has been killed off is A LOT of work
If more people captured network traffic of these services before they're killed it'll probably make REing the service much easier later