It may or may not be a jerk move, business-wise (I have no idea what their original agreement with GoG was when they launched it there), but from a purely preservationist point of view, nothing has yet been lost as a result of this.

(And while it's tempting to say that in the long run GoG would have been better stewards of the old builds than Blizzard, and that would certainly be true for 99% of publishers out there, keep in mind that Blizzard has successfully kept up servers and downloads for games going back to the mid-90s up to the present day. Blizzard takes preservation much more seriously than almost anyone else.)