I really wish GOG could add the source code for the games along with the game itself. Or at least for some of the games.

That would be interesting to see for me as a developer. Many of these old games used all kinds of tricks. But it would also allow for community enhancement, porting to new platforms and whatnot.

I get that this is a big ask of publishers. But many games are basically abandoned. Have no current IP associated with them. The publishers/developers loose little. And it would help with long term preservation.

Well I wish the only obstacle would be to convince the publishers to release said source code of abandoned IP, but in most cases the source code for these old games would be lost anyway. Fun, isn't it?

Sure, it would be nice. But A: Many of the programmers involved are long gone.
B: Many of the companies don't even have the source code.
C: Companies can be weirdly protectionist of their code.
D: What exactly are you going to do with a bunch of Haskell 89'/Turbo Pascal/Visual Basic code anyway?

Decompilation are quite possible, but there's the big, "And then what" that comes to mind. Because with a lot of games, you'd not only be left with a lot of uninitalized code, but you'd also have to do a lot of conversion for fun things like floating point operations, or byte word sizes. Or running on hardware orders of magnitude faster.