I think what Nagoshi's done is establish an effective business model for an ongoing series. I think we can expect things to continue in this manner, with a couple of main titles and perhaps a couple of spin-offs being built on the same engine in more or less annual releases. Then a break as the game's fundamentals are re-evaluated and rebuilt, presumably with a new or noticeably improved engine, and then another series of releases.
Speaking of Ubi, they definitely threw more variation into the Prince of Persia series from one title to the next, to its detriment. The changes in tone were just too drastic from one title to the next (I did really like the art direction of the cel-shaded one, though). I really prefer the smooth continuity of the Yakuza series. It's like the smooth, refined flavor of Suntory Yamazaki 18-year-old single malt whiskey, which is sure to enchant any single malt lover. Full-bodied with spicy cherry-like notes, it smells of toffee and has a lovely, long, dry finish. Uhh... what was I talking about again...?