20 million is super pessimist for RDR2, and also unrealistic to be honest.
25 million is still very low but we are talking now.
Really, 30 million to me is conservate. I don't think people realize how much 600K first week in UK at retail alone are those days... The game will also surely have strong legs.
600k is nothing special for large AAA games. FIFA and COD do that almost every year.
I'm expecting 5m fw and 17m this year. So 13m more lifetime with pc sales seems reasonable.
Also let's not forget that MS and Sony are aiming for backward compatibility next gen so RDR2 will probably just get a graphics and framerate update instead of people having to rebuy the game like with GTAV.