I've seen a lot of people directly comparing GT4,5 and 6 to make a case for GT6 not being a bad game compared to it's predecessors. I believe that's the wrong way to look at it. Look at GT4 in the context of the time it was released. It took GT3 and built on it, not took away from it. The 'career' mode was enormous, and the cars were ALL hugely detailed for their time. The AI wasn't bad for it's time either, and you could choose to do quali sessions if you wanted to start near the front.
Forget online mode, the PS2 didn't have the ability for PD to make GT4 online (technically it had the hardware, but Sony never supported it, much like how it had the hardware to put a hdd into it, but Sony didn't support it).
Now look at GT5 in the context of it's release date. It had been a massive wait for GT fans, who had been carefully fed on a diet of mega hype for the game. It sold like crazy, but in the end it wasn't really a great game compared to other racing games of 2010, unlike GT4, which was genuinely offering something you couldn't get elsewhere.
GT6 came along in 2013, and it felt like an expansion of GT5, and was still offering sub par quality car models, damage, sound, and AI.
Online capability isn't a selling point these days, it's a necessity, as all games have it. I raced competitively online in gt6, and it wasn't really great imo. If you went into public rooms you'd more often than not find yourself racing a room with hugely varying skill levels, and you'd find people rage quitting or deciding to just drive the track in reverse trying to crash into you when they knew they couldn't beat you. Then you could spend the ridiculous amount of time needed to compete in an online league, to guarantee everyone is taking it seriously, but you need to actually dedicate specific times to racing to do that, you can't just pick it up and play. Then you'll have to practice to be competitive, and you'll find more often than not you get to a race weekend, and there might be one or two people you're actually racing against, and everyone else is nowhere near your pace. The race ends with 6 people on track, two of which are on the lead lap, and the rest are lapping whole seconds off your average pace. Then you'll get rage quitters, who thought they'd be competitive, but in the end can't handle losing, so they quit half way through a race. Then you'll get people with poor connections lagging out and causing accidents with their car glitching all over the track.
The above (admittedly huge wall of text) explains why I really have almost no interest in online racing.
1) I am too busy to commit to a schedule, I play racing games for fun, not for a job.
2) I dislike racing against a tiny field of people of all different skill levels, which is 99% of what you'll find online.
3) I'd prefer not to race knowing my race may be ended by someone crashing into me on purpose.
4) I really hate being in a race online and seeing the other cars twitching and glitching around everywhere due to lag.
Decent AI is far more important to me than a well set up online mode.
Also I very much agree with:
I guess the question is whatever the sales numbers, will Polyphony make different decisions if they're lower than if they're higher?
I sort of doubt it.
Looking at it one way, you could make the argument that they're making the best game that they can anyway, so there's no point changing their mind. They can't make it any better.
Looking at it the other way, they're hardly the sort of company that goes out of their way to make sure they satisfy they customer. They make the game the way they want to make it, and you like it or you don't. If they were building games to suit their customer base we'd have a livery editor and full damage by now. And a course maker.
For whatever reason they choose, I suspect Polyphony will go on doing Polyphony things.
this ^
I said no and business as usual.
GT6 came across as a last minute money grab for a system on it's way out. IMHO GT6 should have been targeted as a PS4 title and delayed accordingly. With that said, I doubt GT7 will be any different from the last few games... Carry over a ton of stuff, formats, novelties, etc etc. Maybe they will add a few cars or tracks but otherwise I wouldn't expect to spend time in GT7 any differently than in previous games.
I expect GT will always have an oil change option (like I really need to worry about the oil on my imaginary race car).
and this ^
both snail and GTPES have rating systems. 
