Great subject, and so big. I could write whole day even in response only to main article that opened this thread.
First of all, I dislike modern cars. Whoever says that classic cars are more difficult is not right. Classic cars may require more technique and simply more work to do as a driver, but modern cars requires crazy precision and reaction to be truly fast. Modern cars are usually capable to be "like on rails" (and fastest when driven so) but it is not actually easy to achieve at the limit. Whoever thinks it is easy is craving for simcade. I also suppose lack of enthusiasm has a lot to do with how "easy" the car appears to be, if I am unexcited by GT3 I loose focus, and focus is crucial with such car you just can't let it go. It is probably same with classic cars for others, but classic cars are less demanding on "staying on rails" allows slightly more slip and some milliseconds more to react.
Old cars being difficult is very much a stereotype. Take even 1937 GP car like Mercedes W125. You'd eventually think that it was hell to drive. But it was not. Jochen Mass said that even tires were good, they just were not durable, and you can see in videos that they aren't always diagonal to their direction of travel lol, not at all. Of course not talking about fishtailing at 300km/h. Maserati 250F is difficult till you get in the rhythm with it, even before update in AC it wasn't so incredibly difficult, but I suppose it would be for a person with no basic skill at such driving, no anticipation of whats to come when driving it, and zero enthusiasm with just one thought in his mind "I want to be back on rails in my lovely popular and relevant GT3".
Also classic tracks are great for most cars, as well as modern tracks. With some exceptions, when certain combinations just are not good. IMO modern Monza with its T1 is bad combination with any car. Some combinations might just not always fit the "theme" though it doesn't mean it couldn't be raced in simulation.
A lot of simracers are interested in immersion of current motorsports, and that is main thing, it is just about what is happening now... being in tune with the time and living to this day. I bet if there were a popular modern cars series where they would race street narrow tires, with a cars that rolls a lot, without fancy aero, without electronic helps and overpowered... you get the idea...then it would be popular in simracing too. And perhaps even whole simracing would be more popular, if todays technology wouldn't be so obsessed with laptimes, and would be more about fun and perceiving speed through slides, less on rails action... but technology is moving forward, and giving more effect in hands of a driver would be step back I guess...
I think mastering and understanding simple vehicles helps to understand how car works better. Just take cars that are purely mechanical and you'll have most natural driving experience.
Also having classic cars in true simulation is beautiful way to keep their imprint in history, to remind about them, and some kind of documentation of how they were like to drive. Historic cars in simulation is a document.
Never forget AC Ferrari pool when modern F1 won after 512M, despite modern F1 from previous year already being in a game, and community effort in supporting 512M. That just shows how ridiculous it is about being in tune with this day, with whats popular.
Thats also the reason why advertising works, people has no capability of making choices, they need to be told. Thats mere exposure effect and thats all. So I am happy we talk about classics.