The only way you can get a level that is strictly driver level or skill being the only deciding factor is EVERY variable need to be eliminated.
That means one make only races with setting locked including transmission type of manual/automatic.
All driver aids to be locked with default settings and the Exact same aids at the same level for all drivers.
And controller/ wheel input types would need to be locked to the same input device as well.
Then you would be able to tell which driver was better in that car on that track only, Change cars or tracks the results may be different as well.
Any other type of race with or without custom driver applied tunes or not will not determine driver difference alone and saying it would is a joke.
Tuning and setting a car up is part of racing, there is no series running in the real world which locks all suspension adjustments. It should be the same within the game.
I really don't understand why you keep pointing out that there are other factors in sport mode that effect race outcomes that aren't driver skill related. It's completely irrelevant and at the end of the day, those other factors have a relatively small impact on the outcome of the race, especially as the BoP gets better. Tuning on the other hand can easily make you a second a lap faster if you know what you're doing so to say "well it doesn't matter because you're never going to make it 100% skill based anyway" is an incredibly stupid argument.
In fact it could be argued that choice of cars and driving aids make it
more skill based than a one make now that I think about it. With different options available everyone can pick the option that makes them go the fastest and that suits their driving style, whereas with a one make there is a certain amount of luck involved as to whether the car picked is one you're good in. Ultimately it's still down to driving skill in a one make but it's not necessarily as clear cut as you make it out to be.
Now you might ask, why doesn't this apply with tuning? After all it's just extra options to get a car that suits your driving style. Well there's a couple of differences, firstly the number of options available and the knowledge required to use those options effectively (not driver skill related knowledge at that). With a choice of cars you have 10-15 options to pick from which can be narrowed down quite easily just by looking at the leaderboards, you can quite easily get to grip with the pros and cons of each car and know that you're going into a race as fast as you can possibly be. Tuning on the other hand has virtually limitless combination of options and takes a long time to understand and perfect
The other difference as I've already mentioned is the effect on ultimate pace. With a good BoP the cars will only vary by a couple of tenths in ultimate pace with a few exceptions track dependent as seen with the most competitive cars, so it's not so much a case of "pick this car to go quickly" and more picking the car you prefer and feel compliments your driving style. Tuning's biggest effect however is on ultimate pace, and not on matching your driving style, in most cases as I said before this increase in ultimate pace can easily be a second per lap, and in some extreme cases (such as ovals like Blue Moon) the difference can be up to 4 seconds per lap. So not only is it a case of you have to have a certain tune to set the best times, there is no clear option to pick the best unlike with a choice in car. With car choice it is clearly visible to everyone what car is the fastest but the same doesn't apply to tuning.
Final point is your comparison to real life racing. Yes in real life racing setup is a key part to every race series but there are same major differences between real life and a game (shocking I know). The main difference is there is a hell of a lot of money spent in real racing on paying professionals who know what they are doing and have a good understanding of the cars to set them up for the drivers. The majority of drivers basically do no setup work themselves and probably wouldn't know where to start. In fact in a lot of series the cost of having the best person to setup the car is more expensive than the car itself! So for that reason alone comparing real life series and their rules on car setup to a game, where frankly most people want the options for picking the fastest car to be fairly simple, just doesn't work. That and regulating a stock setup is far easier in a game than it is in real life which also probably factors into why event organizers in real life decide to allow tuning, as well as teams and car manufacturers playing a much bigger role in real races than in game and it not being as driver orientated (although obviously the driver is still the most significant aspect in most series).
"someone doesn't have the desire to work at being better." is exactly the reason a lot of people want tuning, they don't want to work on improving their driving so they want to change setups to compensate. Allowing tuning is a cop out for practicing and doesn't even necessarily require any effort on the person with the setup. All you need to do to gain an advantage with a setup is to know where to find one, same can't be said for driver skill, you can't go on a forum and get more driving ability but you can go on a forum and get a setup. However if you don't know where to find setups and don't know how tuning works then tough luck, you're going to be seconds per lap slower potentially through no fault of you driving ability, and that is a terrible mechanic to have in a racing game.
