I can't make you a legal guarantee of that, but Blizzard will not destroy the copy. It seems unlikely they'll release it any time soon given SC Remastered, but they're not idiots.
I highly doubt it's going to be removed from circulation forever. My guess is it'll be archived internally, the disc itself will end up in their campus museum, and at some point legal will sign off on releasing it to the public.
SC1 is coming up on its 20th anniversary next year, who knows what they'll do for the occasion.
Blizzard already has the source code, in fact it's actively being extended as part of a HD remaster right now.
I have no doubt they are very happy to have the original copy back, it probably means quite a lot of the original developers and I would never dream of keeping it from them had it been up to me.
I would definitely hope they keep the hard copy somewhere safe but history hasn't been kind to game source code.