When, in 20 years, we're hearing that this disc got lost again sometime in the bankruptcy chain of Activision Blizzard, we'll see whether corporate archiving or vigilante illegal archiving is more effective.
Is there any risk that the compiled binaries, on millions of computers and available for free and legal download from Blizzard, will disappear in 20 years?
No. But the source is useful for other purposes than compiling , especially with original comments.
For rarer games, and those shipped on DRMed/copy protected media, it's certainly possible to get into a situation at risk of losing all known copies of the binaries!