My main problem with classic cars is the following: I don't have room for an H-shifter and hell'n'toe-ing with paddles is so unnatural. Brings me out of the immersion.
You're absolutely right, I used to like classic cars before, but never fot into them because I've used paddles to change gears, or had to use auto clutch, blip and shift because I did not have a clutch pedal. These cars were cool, nice sounds, great design, but I couldn't love them.
As soon as I got a 3 pedals set and a H shifter, I understood immediately how insanely great they were and how much as I have missed all these years without the proper equipement.
To enjoy classic content you need better equipment, and many are not willing to spend more money for a hobby they are not sure will bring them more enjoyment than the combo of cheaper equipment and modern cars. I discoved the classic car pleasure because I found a unbeatable sale for a g29 with its h shifter. Without that opportunity I would not have invested in a 3 pedals set and a h shifter.
Another thing about classic content is what is considered as classic by different generations of players : I watched formula 1 and other categories on tv in the late 80s, 90s and early 2000s, so for me it's not really classic content. Cars from the 60s, 70s, and early 80s are classic cars for me. Before the 60s they are antiquities . Therefore I am more interested in the cars contemporary of my youth, the 80s and 90s and early 2000s, when I watched much more races on tv and started sim racing, and those from the 60s and 70s, as they were still shown in movies and documentaries.
But for someone currently in his 20s, the 80s, 90s and early 2000s cars are classic cars. And those before are antiquities. And the late 2000 cars are the cars from his youth.
The definition of classic cars depends on the age of the simracers and I assume it's really hard to appeal all of them. A partial solution has been found, including one car for various seasons and not providing a full championship for one specific year (and I assume it would be difficult anyway to get all the licenses to do so). By doing that, any simrzcer can race what he considers a classic car. But he loses old racing rules, he races single make competitions, often on modern tracks... Project cars 2 had done a step in the right direction, specifically for the GT series, providing several cars for each period and a few classic tracks. It was and still is imo its main quality.