I sort of think that "Modern" means "still in its heyday" where "classic" tends to mean "Thems were the days".
For instance:
US manufactured cars = Pre 1973-ish, with "spots" here and there for the "Fox body Mustang" or... well, I can't really think of anything else... We had "great cars" then we started building a whole lot of crappy ones.
Japanese manufactured = 1970-ish till about the mid-nineties. You had Mazda Rotaries, Nissan inline-6's, Datsun 510's, Toyota Celicas, etc etc etc. Then as the Japanese supecars peaked in their twin-turbo-ness they all kind of fell off... No more Supra, No more RX-7, No more 300zx. All tried to make returns (Z and RX-8) but no REAL success. (they don't make them like they used to). The exception can be made for the Hondas that were zipping around after Fast and Furious. Those definitely had a following that fell off. I'm not entirely certain that the Japanese are making "crappy cars", but they've definitely lost the popularity (just like there are good American cars being made, but they don't have the spotlight like they used to)
British: Jaguar e-type days, and Austin Healey Sprite days. Their "classic" days ended when the roadster died.
Italian: Fiat 500 and Alfas pre-fiat. For me the Italians fell off when Alfa went FWD with their 164. bleh That's not to say I don't think they're "not sexy" but I cant bring myself to spend money on one. They're definitely on the right track with the 4c, but they have built some awful crap (imho)
This brings me to "why the Miata isn't a classic yet". Because Mazda hasn't killed it yet. It will be a classic "eventually" but until it sucks, there wont be a "classic Miata".
Which also brings me to the Koreans: I think we may be seeing the advent of the "Korean Heyday". They're making some pretty nifty cars that are affordable. Once they go to crap and the Chineese take over (they'll be next, I'm sure) then we'll have Korean classics (and so on)
I think every car manufacturer follows a lifecycle:
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Startup = cheap crap no one buys (think Kia 10 years ago)
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Getting it right (and getting popular).
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The "bulletproof reliability" and "car culture" badges (when they're on top of the world)
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Back to crap (and now expensive because they're playing on their former popularity **cough cough TOYOTA!)
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Back to the roots: Think Jaguar. Lots of crap, now we have the F-type. Or Alfa, lots of crap now we have the 4c.
Mazda is an odd one. They always seem to have a "cool car" in their lineup, but never get credit as being a manufacturer of "cool cars"...