~1.9m people is still a large userbase. I think the biggest roadblock is logistical: AoE3 was sold outside of Steam, on the GFWL Marketplace and at retail, the former of which is now dead while the latter doesn't use GFWL, so in order to be fair, Microsoft would have to develop a website that analyses GFWL accounts/retail keys and grants a Steam key for the Complete Collection, and also set up a support system to account for those who have a purchase receipt but either can't log into their long-forgotten GFWL account or no longer have their retail copy. It doesn't help that the game wasn't available digitally anywhere upon release.
