I actually read your post unlike most people probably and you have some decent reasoning.
I still think 20m is very, very optimistic though. Even GTA San Andreas didn't sell 20 million (and yeah I'm gonna mention userbase) and the PS2 userbase was huge.
Here's my prediction (more like possibilities)
20m+ Sales = 0.5% chance
15m+ Sales = 7% chance
12m+ Sales = 37% chance
10m+ Sales = 51% chance
8m+ Sales = 75% chance
5m+ Sales = 100% chance







