Whatever the risk was. It would've been worth it. Gran Turismo sells on its name alone but Sony let GT fans wait for four years before Gran Turismo Sports was released. That is damn too long even considering Gran Turismo's PS3's long cycle of development. The mindshare of Gran Turismo for this gen is in critical conditions. Forza and other racing sims caught up or to put it accurately, Gran Turismo has gone down because of its long absence in the generation of PS4.
If Polyphony is still sane, they should know they can't keep resting on their laurels. The prestige remains but they are in trouble of losing sales and mindshare as the racing sim genre continues to shrink in market size. They do not even have have other IPs to fall back into if Gran Turismo flops so they have to be smart or evolved the game to continue or reclaim their dominance in the racing sim genre.
This is still great though. Sony have reasons to be more critical to Polyphony's autonomy. Their sales underperformance and naivete of this current generation should lit a fire under their butts so they won't make the same mistakes again.
Because spending time porting a GT would have been welcomed with open arms by fans over a new entry?
Like there is no real benefit whatsoever to the port unless it was day 1 on PS4 with the original PS3 release.
I don't think a late GT6 port is even remotely as beneficial as you claim it to be. The answer wouldn't be a port but rather GTSport releasing sooner.
And last I checked, GT still outperforms Forza in terms of sales. Likewise the GT brand is fine.