Totally agree with both of you. I have enjoyed prologue a lot, in fact its one of the few games I bought for the PS3 (I bought my PS3 for it). I'm not questioning whether it was worth it, I still maintain that relative to the size of a full release, its nothing more than a demo.
The issue that I have with it is that the huge revenues from prologue directly results in a prolonged developmental time and an anal pursuit of perfection, it turns the game into an art project. Sometimes trying to be commercially successful is good (things that most people want are prioritized). I don't see it encumbering the innovation aspect of GT (to a significant level).
I still remember getting blown away when GT3 came out, there was nothing like it. I would prefer a less than perfect GT (with ~150 cars and ~15 tracks) within 1 year of a console launch. GT4 was better in so many ways but the zenith of my excitement playing GT4 was only half of what I felt playing GT3. A version early on in a console life cycle would be great for both the fans and the console.
PD know how to do things right, they still need some work in doing the right thing department. They need to know what 'too much' is, time should be an important factor in the development process. If it weren't for the sheer brilliance of PD, the series would have been down in the gutter a long time ago (considering their proclivity to delays).
Being a GT fan requires a lot of patience.
Oh, wait: 
