ribrsiq: 0- Do Sierra On-Line still exist?
1- Does anyone provide proper support for these games? IE, bug patches and so on? At the very least making it compatible with modern OSes without using 3rd party software?
2- Does anyone even still have the source code?
3- Would anyone understand said source code even if it were, by some miracle, located without extensive study?
4- Do the original developers/publishers get a penny out of what you pay for them? Or does all the money (apart from whatever GOG or Steam, etc. get) go to someone who acquired the license almost-certainly unintentionally by buying the remnants of a company (that had bought the remnants of a company)*10^n of the original publisher?
There are good reasons that there exists something called the public domain. Go read Free Culture or something!
0: No, but the rights to their games have been passed on to other parties.
1: GoG does if they land a release. Otherwise it's up to the current right holders to decide. For example, Apogee still have forums open for their old games where you can get support.
2: Probably.
3: Unless the source code was written in some ancient alien language for some reason, I doubt it would take much to understand it. Every time source gets released for an old game there's mods and enhancements for it out within a month.
4: Depends. Mostly not if the game is old enough.
There are good reasons that there exists something called the copyright laws. Go read How To Make Money or something!