It's not really "fair" to judge GT6, just by having played GT5.
--However-- It is "almost" possible to *know* exactly what GT6 is like having played GT5. It is effectively a major update, GT5 Spec III.
I would argue the new content is worthy of a new game tag, but as far as actual gameplay, and the overall ownership experience of GT6, is exactly like GT5. Very few people that didn't like GT5 will like GT6, and vice versa for those that do.
The problem is that your assertion is entirely based on assumptions that have no way of being validated. Especially since it's based on your personal experience.
For some, the only Devil May Cry worth playing is the first. Others glean a common thread through the series.
For some, Final Fantasy stopped being an RPG and became an interactive movie when it became a PS1 series. For others, it grew up from the 16-bit era.
For some, Dream Theater died when Kevin Moore left. For others, Jordan is king of keyboardists and arrangers.
For some, the only Bond is Sean Connery. For others, the series didn't come to life until Pierce Brosnan or Daniel Craig came along, or Mike Myers... oops, sorry.
To some, Jesus is a swell guy, to others, he's God.
Okay, I'd better quit. Anyhow, there is a discreet difference between GT5 and 6. I didn't care for the "dinkmobiles" in GT5, any more than I did the previous GTs. But I like them in GT6 a lot. I like every car. GT6 really surprised me, because the GT Academy demo was nicely different, but subtle. The cars came to life in GT6.
So see, you can't judge anything for anyone else. They have to do it themselves.
Try this for fun; pretend to explain GT 6 to a friend that has played GT 5. Be objective and not embellish in your own fandom.
Did that sound like a new game? When you lump all things together and treat them equally it sounds more like an update to me...
Yes, but don't just limit this to GT5 and 6. "GT1 was this crazy game that dared to put you behind the wheel of street cars! Who'da thought it? It broke the racing game world wide open. Then GT2 took everything that made it great and added tons of cars, tracks, and events to it..."
Thinking about all the games, only GT 2, 4 and 6 stick out to me because they expanded the previous title so much. Well, and GT5 because it made Gran Turismo so incredibly realistic. But describing any game in the series is kind of a rewording challenge, because the core remains the same.
I know some are a little bummed because we got another small initial game like GT5 was. But the difference this time is that GT6 is built to be expanded with new features, kind of like a cellphone. And as GT6 is expanded, it's going to be creeping towards something like GT7, just lacking goodies that PS4 will make possible.