Except as a buissness they have to think of things like how many of those people will actually come back and play those servers like they said they would? How many of those people would actually play/pay for more than 1 month? things like that. Imo it's likely not remotly worth the monetary risk to them to go back, reprogram older versions of the game (even if they have the old patches, they are unrefined and would need to be polished up a bit), run extra servers JUST for legacy reasons, likely hire some extra support staff to handle the support needs of those that play on the servers. So on and so on.
If it were my company, I wouldn't do it either.