Blizzard keeps several nearly-dead servers and even avoided closing these down by coming up with "cross-realm technology" - to avoid the bad PR that shutting down servers brings - so it's not that preposterous to assume that it's not that expensive (by Activision-Blizzard standards, that is) to have, keep and maintain these servers.
So I do indeed feel that having two legacy servers, per region, wouldn't be the end of Blizzard.
Do note, that I'm not saying that this would be an awesome investment for Blizzard, by any stretch. As we got no way of knowing exactly how many would actually adhere. Merely stating on the fact that I do think Blizzard would be able to pull it off.