Nintendo is king at managing IPs.But if that was truly the case, then how Nintendo is able to keep winning over the Japanese market while also relying on iterative sequels for series like Pokemon and Mario Kart and Smash etc?
It is more complicated than that, though I agree Japanese publishers are playing their cards far too safe these days.
They changed a lot of Zelda conventions with BotW and made the game explode in popularity.
Mario Kart, Splatoon and Smash are MP games at core so iterating isn't an issue, just like it isn't one with Monster Hunter.
Even 3d Marios have different formulas and therefore always feel fresh (sandbox like Odyssey, 3D like Galaxy and 2D/3D like 3D Land/World).
But when you take a franchise like Yakuza, which is Single player, and you get the games at a rapid pace then decline is to be expected.
More generally these are experiences with little competition. Who is making games like Mario Kart, Smash or Mario ?