Since I agree with your pro's I'll just comment on your con's:
1. Yeah a 5 year wait maybe will put off some people....but I doubt it will do major harm as a lot of GT's fan base is pretty semi casual and a lot of GT's market will buy it no matter what. Also many people will buy it just because ITS FINALLY HERE after the wait!
2. Currently it looks like a November 30th - December 5th release....between that. If its announced this week SONY have ample time to market it massively. REMEMBER we are in the holidays.....people will have impulse buys
3. Sure we've seen change in gaming habits....but it isn't that extreme because MGS4, RE5, FFXIII still sold very well well in the West. And Gran Turismo is a completely different beast in itself....its a game that is known as the leader of its genre....trends may change, but GT5 itself may end up changing trends.
4. Having no porsche won't bring down sales....most people won't even know this. "too hard getting cars"......thats always been there in GT, nothing different from past games really.
5. Don't see this as an issue......70 track variations itself is a huuuuge.
6. Yes it may be weaker in Japan....but lately PS3's growth in Japan has been huge compared to before....GT5:P came to PS3 in Japan when it was a wounded beast.
