One thing that nobody has mentioned is that owning/driving a sports car isn't as much fun as it should be due to too many stupid drivers and stupid driving laws.
If you try to rapidly manuever around/through traffic, some idiot will purposely block you and/or call the police to report a "Crazy driver".
And what's the point of having a sports car that can safely take a highway curve at 120, if the speed limit is 70 or less, and a generic car/truck/SUV is capable of safely taking the same curve at 90?
Some other thoughts on the various topics discussed on this thread from the perspective of a happily married "millennial" (hate to identify as that, but it's technically accurate) with a couple kids and a bunch of cars:
A lot of young people (and people in general) are just not interested in what makes cars or anything else function, as long as they can operate it well enough to serve it's purpose. I, however, have always been interested in how things work - cars, guns, mowers, electricity, appliances, plumbing systems, the stock market, computers, etc.
I bought my first car (used Honda Accord with 195K) at 17 and my first and only new car (Ford F150 XL RC/LB 2WD 3.7L V6) at 21. Still have both roughly 10yrs later. I've thought about buying another new car at some point, but not many options that appeal to me that I could realistically afford/justify, considering that I own more than 10 vehicles currently. I really want a red Honda Accord EXL V6 2dr Coupe, but they stopped making those in 2017, so I'll probably get one in 2027 when I can afford it. I do like the 2018 sedan also.
I absolutely HATE, HATE, HATE crossovers so much and can't imagine ever owning one.
Another thing I do not like with newer cars is all the un-proven tech. I know a Honda engine will go for a looooong time, but how long will their "lane sensing" reliably last? 10yrs? 5? Or Ford's engine stop/start feature? Might work OK now, but how will it affect engine longevity? Last longer due to less time spent running? Die sooner from constantly restarting?
My wife (just a few months younger than me) got her license at 23, several months after we got married, 5yrs ago. My little sister (several years younger than me) got hers at 16 and has put about 80k miles on her 06 Odyssey in the last 3yrs. My other 2 sisters didn't get their own cars until the ages of 22 (license at 19) and 30 (license at 17 but shared car with husband until they bought their house at 30). Brother bought car at 17 license at 18. None of them own a sports car or ever intend to. Closest thing to a sports car is my BIL's 08 Mazda 6 sedan with an auto trans lol.
Closest thing to a sports car that I own is a 2dr 1990 Buick Century 3300 V6, which I have occasionally loaded my kids and carseats into. Interestingly, it's easier to load the carseats in the 2dr than in my 4dr sedans, due to wider door entry and seats that fold forward.
Also, I am now the single income earner for my household and I earn a good amount below the median income for the area (about 200% of federal poverty line) working as a maintenance tech and still managed to pay off 4yrs worth of student loans (no degree), buy a house (down payment from savings from when wife was working before kids), pay for mortgage, gas, 403 (B), utilities, diapers, formula, health insurance, groceries, etc, pay for my truck, and still be able to buy a fleet of old beaters. I would argue that income is not the issue with Millenials, it's how they choose to use that income that prevents them from being able to buy a new sports car if they want one.
Millenials and GenZ or Gen0 also tend to be more eco-minded, and sports cars (or automobiles in general) are not generally considered to be eco-friendly. That may be a contributing factor to sales declines.
Many people in general tend to not plan ahead and see no use for having a backup vehicle. They would rather spend more money on buying newer cars, and when one starts having issues, they sell it and buy a slightly newer car that then starts having problems and they keep repeating that cycle. If they had a decent backup vehicle, such as a sports car, they wouldn't need to be as concerned about an issue with their main DD, as they would already have a vehicle and could better plan their car purchases without being pressed for time because their DD needs a large repair/gets wrecked/ stolen etc.
Sorry for the long post.