The proper choice from PD would have been to remove the REASON modding became so popular. That reason being more extensive tuning. Limited tuning contributes to the imbalances in the game, so why not address this fundamental flaw of the game and give all cars the same tuning range and let the player's tune the cars they want? Why limit the ride height range for each car? Instead of ruining my modded car, why not just increase the ride height range so a suspension data swap in unnecessary? Same with transmissions. Why can't a 69 Camaro have a 6speed, or 7, or 8?
They made an arbitrary choice in the original design that conflicts with every player's desires (even the die-hards). Every player who tunes their cars can appreciate having more choice to achieve that better balance for their car. It would have increased the number of competitive cars. That simple change would have addressed the problem and improved the game. Win/win. Instead people that didn't mod get nothing and the modders lose something of value to them and continue to search for way to get it back.
You're so far off base it's hard to know where to start here, so I'll go right to your core misconception.
No one at a game development company of this profile is making any arbitrary choices. Every game design element is scrutinized and implemented for specific reasons and goals. I'm not sure if you've noticed; but this is the 5th game in an iterative series.
The original Gran Turismo game is philosophically and functionally identical to every other numbered title to follow. There is no story in a simulator, they just use new technology to improve the experience.
Suggesting that infinite tuning is something everyone wants and it's lack is a fundamental flaw in the game is just ignorant of history and a narrow minded generalization. If what you say is true they wouldn't have sold enough copies of GT2, GT3, GT4 to even make GT5, let alone continue to develop GT6. They have added in more levels of customization (there was no paint shop on the PSX for one) as technology allowed and their customer base grew to include more diverse automotive cultures. However when someone buys a "Gran Turismo" title they get Kaz and PD's idea of car culture. Any creative content will be funneled through the lens of the developer; which in this case values car variety over car customization. That's just the philosophy of the title, if you want infinite variability and feel so strongly that everyone else does... make a game that is shaped to your vision.
Now that that's out of the way I suppose I'll play devils advocate with the details.
*
Limited tuning contributes to the imbalances in the game
Shoving a V8 engine into a Fiat 500 doesn't create an imbalance? Other's would say that limited tuning retains the character of each car so that each one is more of a unique experience. The whole point of having hundreds of cars is to create variety after all.
It's also possible that PD thinks limited tuning balances the game by removing the potential for equal drivers in equal machinery to have wildly different performance due to setup changes. Philosophically they have always been trying to keep GT an accessible
driving game, not an engineering tool. Simplified tuning was one of the draws of the early games which were competing against high-power PC simulators which required an engineering degree to manage car setups down to tire pressures, caster, and steering rack ratios.
-
*
Why limit the ride height range for each car?
Most likely programming constraints or the mechanical specifications of the vehicle. There are plenty of reasons to limit a specific tuning element like ride height. Should the game allow someone to drop the car within a mm of the pavement if the geometry of the suspension or wheel well clearance would never allow that kind of modification? From the programming stand point it is certainly a lot more cost effective and efficient to determine an envelope of "usable ride height" and apply those constraints to every car.
-
*
Why can't a 69 Camaro have a 6speed, or 7, or 8?
Other's would say once you do that it's no longer a 69 Camaro. Goes back to the core philosophy of the game, they obviously want to maintain some sort of character from car to car. Do they add a 1954 Gullwing to give people an avenue to drive an old classic that is monetarily out of reach, or so people can put a 300SL body on a GTR chassis?
These are also advertisements for the auto manufacturers and they most likely have some say in the use of their products. You may have noticed that Corvette C7 which is completely locked down. Did PD just make an arbitrary decision to do that or did GM say they wanted the car to be a "virtual test drive" mandating that it couldn't be tuned?