The rating system needs splits to work properly, if you look at one split you're right and if you finish where your car number says you'll just continually lose iRating, but if there are splits then you might be car #15 of 15 in one race but car #1 in the next, but a lower split, and if you're good enough to 'deserve' a higher iRating then you will slowly creep upwards (or very quickly if you're really good), and then eventually you'll find it stabilises around a certain level.
For me the biggest issue is the general nature of it, in a Skippy or Radical (and given lots of practice as I've not done much for a while) I can be a pretty quick driver, I've qualified top-10 overall in a Skippy week before which was awesome, so with those cars I get my iRating to around 2.5-3k or so and could probably push that higher if I tried to. But put me in the Star Mazda and I'm terrible, I'm nowhere near that level yet my car number/rating implies I am, so if I did a season in a car like that my iRating would drop like a stone.