While I do agree the Yakuza series is going to be over milked if they don't make other games between sequals. It isnt a series I want to see end, I would much rather it took off in the West and we got one every 2 or 3 years.
In little over 5 years , we've had (well about too) 5 Yakuza games , that insane , when each game is like a 20 to 30 + Hr RPG, and not even a single issue game like (FIFA) .
It's a sure fire way to limited new idea's , really improve the series , and kill the Staff/Team and see them leave SEGA. The staff working on The new Monkey Ball or VAL II have had more development time , its madness imo .
It's going the way of Sonic , even to the point of introducing new playable characters , in a effort to hide the lack of anything new.
I know its a great series for SEGA in terms of marketshare in Japan , but the Team got to be given 2 to 3 years development time , for each new Yakuza game .