Not that this hasn't been touched in this thread already - but I would have to imagine that the reason some old cars remain on the roads and others do not has to do with the conflux of a few factors:
1. The initial price of the car new, which is related to how many were actually produced in the first place.
2. How well the car was built, from the engine to transmissions to electrics.
3. The power of the Brand. This is reflected in brand-loyalty of the customers to a manufacturer (or even a specific model), or what principles/concepts that brand embodies to a customer(s).
This also has to do with the amount of dealerships (past or present), the amount of mechanics knowledgeable in the car(s), the availability of parts, and their ease of repair.
4. How well these cars were taken care of, and where they tended to be distributed related to the environment (eg: niche cars popular in the rust belt, would. well, rust).
You see 30 year old Volvos driving around because they are simple, have bulletproof engines and were quite numerous to begin with. Same with old Hondas and Datsuns. Parts on these cars are/were reasonable.
You see quite a few Saabs because they also were well-designed and had great engines. They, however, had certain deficiencies over time that certain people overlooked due to their quirky niche in the car world. Parts are also fairly reasonable and there is an established network of dedicated mechanics to service them. This overcomes the fact that they have always been somewhat of a niche car manufacturer (so not too many to begin with).
You don't see a lot of old Jags, because they were expensive, lost value quickly, had famously poor electrical design, and not many people could afford to work on them. Alfa Romeo probably falls into this category also.
Cars like Peugeout (and Alfa) also left the US market, stranding their cars (obviously not that numerous here to begin with) - with more difficult to access parts and less mechanics to fix them.
I think MGs and the like survive despite being in a similar boat because they were roadsters, which seem to be a favorite for restoration - though that's only a guess.