A lot of your concern of older games has been fixed over time by the development of emulators and wrappers. Multiplayer (and live DRM) components of modern games can't be easily RE'd in most cases. Especially with services like Xbox Live the most helpful data is actually packet captures where one can actually go through and replicate the protocol used.
I honestly would challenge you to find games from the mid 90's+ that won't work on a modern machine. Many times if there's a feature that straight up doesn't work it's due to online services being discontinued. There's been some work done to rebuild services like GameSpy or Westwood Online, but once more forms of encryption and anti-cheat or DRM were implemented in these sorts of services, the harder it's going to be to preserve their experience.