If PD don't want smurfs, then GT1 shouldn't be exclusively less flexible, (incredibly) longer races with less variety. It's incredibly stupid game design to have such wildly different race settings avaliable for the same race - as if you are better suited for the GT2 format then you are essentially encouraged to drop down to partake in it.
What if you're busy on Saturday evenings and cant make the hyper specific GT1 time slots? What if you've a child that might require attention, or family that you want to spend time with instead? It's a long race, 90 minutes is a gigantic amount of time to have to sacrifice on a weekend evening and anything that could oppose that creates a gigantic waste of time.
Or.. they could tank their rating and hop on the GT2 race on Saturday morning, the race when they have downtime or the race when their family is asleep or preoccupied. It's not hard to see the appeal of doing so when GT1 now demands so much of you.
GT1 league is only A and above - It's not like this is exclusively the best drivers in the world duking it out for a spot on the world stage. A is achievable by pretty much everyone if you enjoy racing games and if you're pretty decent at them - and when you can get that rank from doing 7 minute Race B's every couple hours every other day, being thrown into the 90 minute endurance category is a pretty drastic increase.
Or.. you could drop to the half hour long race in GT2 league, it's right there. And hey, you do like to race Gr.4 cars, and you wouldn't be able to otherwise...
tldr; PD are pretty much softly encouraging people to 'seal club' by making the races so different and forcing people into a massively longer race that only so many people are suited for. If there's a more suited race format for them avaliable just by dropping a DR bracket, why wouldn't they?