Also I don't see the user base mattering much, the differenc betweene FFXII and FFXIII is 400k. The different between userbases was abut 10m.
Looking at that the difference between PS4 and PS3 by time of XV's release will surely be insignificant, so FFXIII's 1.8m figure should be a realistic target for XV. The game seems more appealing I think it will actually do over 2m