There are two things that make Polyphony Digital different from the usual game developer.
- Kazunori Yamauchi has a passion for motorsports and cars in general that a number of gaming journalists have remarked on, not to mention racing skill and acumen. Many attribute the flavor of the Gran Turismo series to the head of the team.
- Polyphony Digital is apparently as much like an extended family of sorts as a game company. There are close teams like Insomniac, but PD seems to be unique in this regard.
If The Kaz left Polyphony, the game series is still in capable hands, but I could see that an essence found in previous games might begin to thin out, and Gran Turismo become another "same but different" series.
By the way, that was a pez poor "joke." I'm sure that Forza fanz would be delighted at the poke that Forza would benefit greatly if it just had someone with actual car passion like Kaz at the helm.
Heck, you can't even mention the unsavory nature of Microsoft without some Forza folk throwing a tantrum about it.