BTW turrican 3 source code & tools is already available (that's the only example of a commercial game I know of!!). Changing something and rebuilding the game would not be a piece of cake even with all the data!
But don't think that giving away 68000 code is harmless for companies. Don't think that recoding a 16-bit game in C++ so it is identical to the original is easy (even a 8-bit game remake is tricky). I've been there it's NOT easy.
Say, if the Bitmap bros would release sources for Speedball 2, we could figure out all the A.I. code and transpose it to C++. For the rest I agree it would be faster to recode from scratch (menus, animations...)
If companies want to preserve their original algorithms (A.I., speed/difficulty settings) they have no other choice than keeping the source code for them (and risk to lose it eventually)
See Putty Squad. Someone has the disks, could release it as a freebie but has no interest in doing this, plus he would take the tiny chance that the game would sell less on a yet-to-be-coded mobile version. Stupid but that's business.