My take: not the case. They had an opportunity to use a next generation A.I. and embraced the chance.
But when there is a cutting edge/game-changing opportunity, a wise company accepts the challenge. Given the chance to have Sophy created vs. plugging away on their (collective) lonesome? I believe they made the correct choice.
More on this in a moment, but I think you're missing something here. Sophy isn't tacked on to the stock AI; Sophy is separate. Also, Sophy has proven to be way better on the track than the best drivers in Gran Turismo repeatedly; during the Drive Together limited event for the most part the only way to beat Sophy one-on-one was to start ahead and either block the entire race or try to force the other car off track/force it to drive off track.
In this
wonderful feature on GT Planet, there's a frank conversation about Sophy. From that article, Dr. Wurman says the following:
" Many said that it provided a game-changing racing experience and that racing GT Sophy resembled the best experience they had racing other humans in the online mode, but without the hassles. Many also requested the ability to turn down or modulate the skill so that they could find a level that matched their playing skill."
Here's where I think you've missed something that (gladly) PD didn't.
Sophy is now far more accessible, because the game is intended for a wide swath of users.
It's easy to be disappointed that you can't have the Sophy that beat the brakes off everyone from the Live Event circuit but remember it's mathematically likely you aren't at that same level and - therefore - would fare much worse than they did.