This is an interesting thread because of course we all have differing hopes for what will improve in GT6.... But the reality is that Gt5 managed to sell 10 million copies. There is simply no measure where 10 million sales is anything other than a big success for the game, no matter what the shortcomings.
To put that into perspective, Forza 4 is probably rated critically by reviewers and (impartial) sim fans slightly ahead of GT5, but Forza sold around half as many copies as GT5. And despite being released a year later, Forza's rivals time trials get far fewer entries per month than you see in GT5's seasonal time trials every 2 weeks (anecdotally many rivals events manage less than 10,000 entries globally in a whole month, more people gold GT5 time trials within 24 hours of them being added), so there really is a far larger online community for GT5 2.5 years in, compared with the Forza 4 community tailing off dramatically within less than a year, and still dropping fast.
There is a real risk that GT6 sells considerably worse than GT5, of course. It depends how many people purchased 5 for the "look what my PS3 can do" effect, since very few of those are likely to repeat purchase 6. So we might have the bizarre situation that GT6 is critically well received, fixes a lot of 5's issues, but has considerably lower sales nonetheless!