Although the term "milking" gets thrown around a lot here, they haven't made a bad Yakuza yet, and IMO each one has improved upon the previous one in huge ways. It's not milking... it's more like "strike while the iron is hot."
I disagree. If you're releasing 1-2 games of a series each year, you're over-saturating the brand. It's good that they are improving with each title, but if you put up screen shots from each of the games per each system, I wouldn't be able to tell them apart. The yearly cycle though with Sonic, Call of Duty, Madden, etc, really drains the excitement behind each future release.
I do agree that it's great to see a series have success like this, and I wish more franchises could release more titles often. I wish I could live in a world were they released a Jet Set Radio, Daytona, Eternal Arcadia, NiGHTS, Zelda, Panzer Dragoon, Guardian Heroes, Valkyria Chronicles every year and complain about how the quality has gone downhill and that the next one might finally be the one to break the cycle.