I concede that 'loved' might be an overstatement, but I'm basing that judgement on Wikipedia's summary of critical ratings, which list it's aggregate score as 82/100, with only Electronic Gaming Monthly and GameSpot awarding it less than 8/10 or equivalent, of the critics listed.
Metacritic gives a wider range of reviews. The average is still about the same, low 80s, but you can see the spread better.
Looking at Gran Turismo 5's aggregate score of 84/100 and it's (arguably) subsequent sales, I'd say GT6 has done slightly better than "pretty all right", it's still considered a top level game, and I genuinely do feel that were it not released on a console at the end of it's lifespan then it's sales wouldn't be far off those of GT5.
Don't get me wrong, I'm not saying the reviews are bad. But there's a big difference between a game that gets 81 (or 84, if you want to be generous and take the Wiki number) and a game that gets 95s, like Gran Turismos 1 through 3.
Mid 80s are generally a good solid game, good fundamentals, worth playing. I have plenty of games like that in my collection, and most of them I enjoy and will never sell. But it's not something genre defining or otherwise outstanding, which is what GT has been in the past. In terms of game scores in general, an 84 is very respectable, and most developers would be very, very pleased to make a game that good. But for PD, it's a step backwards, and it's a continuation of a trend that has been going on for some time. Since GT4, really.
I hate to say that because GT4 is usually held up as the pinnacle of GT-ism, and I mostly agree. But purely numbers-wise, the slide started with GT4.
I don't really buy into the whole sales affected by it being an old console thing for one reason, GTA V. I think across PS3 and 360 they sold something like 30 million copies? Staggering numbers, and it shows that old hardware or not, if you make a really desirable game then people will buy it anyway.
While PS4 and Xbox One likely have some impact (nothing happens in a vacuum), I don't see them having nearly enough penetration yet to be significantly affecting GT6 sales. Not to the point of losing millions of sales, anyway. To me, GT6 rises and falls on it's own merits. There's plenty of PS3 owners out there still.