I would love for Diablo classic to come to GOG, as it was the only cRPG that I heavily got involved with in its online competitive & social element, which were the bnet-hosted chat-rooms and 4-player co-op games at the time. Guilds, ladders, the whole bit. Really trailblazing that way, and great fun for a young, depressed teenager with few other outlets.
Foolishly, I let my legitimate, bought-and-paid-for disc copies of Diablo & Hellfire get thrown away or otherwise lost years back after I'd put them to rest after getting burned out on the game. But of course, nostalgia strikes, and lo, you couldn't conceive it at the time you thought you were just shorning yourself of excess belongings; books, music, movies, games you had no further use for or interest in.
Blizzard would make a tidy sum, I'm sure, re-selling Diablo on DD channels for a certain demographic of gamer, of which I fall squarely in. I doubt classic Diablo/Starcraft is of much interest to the generation of gamer born in the 90s (and even later) anyway.
Of course, maintaining control is in some cases more of a corporate imperative than any sensible alternative. Even if it means letting one's back-catalog wither on the vine.