Barozi on 04 April 2013
You're even ignoring that many exclusive sequels have lower sales.
LBP > LBP 2
Resistance > Resistance 2 > Resistance 3
Crackdown > Crackdown 2
Forza 3 > Forza 4
Halo 3 > Halo Reach
Warhawk > Starhawk
Super Mario Galaxy > Super Mario Galaxy 2
and much much more. If a multiplat sequel to those games would be on par with the latest game in the series I'd say it was definitely worth it as the trend was going down.
EDIT: Doesn't have to be first party games either. But there aren't many examples of third party exclusives that stayed on only one platform.
