I understand what you're saying, but it Capcom even capable of making two Monster Hunters concurrently with good quality?
Probably not. People have been ignoring that point. All of the Portable games are based heavily on the equivalent mainline game and Generations is heavily based on 4 and reuses assets from all the past games.
They were never an entire separate series.
MH3rd is still the best selling Monster Hunter to date?
Yes, although the 4th Generation of games as whole is more successful than the 3rd Generation.