Gran Turismo is and always has been about the cars. The racing was only ever arcade-like. It's a driving game - what people take away from driving is a very individual thing, so the better GT can cater to that individualism (something Kaz is very keen on), the more people will get a satisfying driving experience from the game.
On Reason 1: GT5 has plenty of "social" features already. It is a social game, in as much as people have been playing GT socially as long as it's been around. Again, individualism is the key here. And, once again, "free roam" is not necessarily NFSU2.
On Reason 2: One of GT's historical "weaknesses" has been its focus on the cars, to the detriment of the scenery. "2D trees", low-res textures, missing props etc. You are putting up limitations where limitations don't exist - the course creator themes have dynamic scenery superimposed onto a few pre-made fixed terrains with a bit of fixed scenery on them. Look into procedural generation, it's been a feature of computer games practically since their inception.
What GT "does best" is clearly not the racing, unless, perhaps, if you go online, at which point it's more-or-less a social feature.