Solid colours in an OEM situation (in the majority of cases) do not get clear coated, it is not necessary.
Metallics or pearls get clear coated because they are only 'base coats'. They are not a 2 pack product, they are a single pack product that dries fast, generally has no shine and definitely has no protection, hence the need of a clear coat.
Clear coats and solid colours in most quality 2 pack paints generally have the same 'make up' (for the sake of this conversation anyway). Meaning they are both equally as resistant to scratches and UV, or very similar. So by putting a clear coat on a solid colour is not going to make the surface any tougher. They have the same 'toughness'
Yes in the industry we have had people request to have clear coat put on their solid colours because it does give the illusion of more depth, but when we do this we use a 'base coat' product for the solid and then clear coat it, we don't generally paint the vehicle in a 2 pack solid and then clear coat that. There are good reasons for only using base coat/clear coat in a solid situation but no need to get into that here.
As for the cost of getting a plain solid colour car or a metallic/pearl, then yes the base coat/clear coats are dearer because there is more product and it is all done twice. As a general rule (products vary but to keep it simple), to paint a car in a solid white or red or whatever, you put 3 coats down and its finished.
To paint the same car in metallic or pearl you put 3 coats of the base coat colour down and then 3 coats of the clear coat on top.....hence twice the amount of work and product.
If someone is getting rust issues then I would suggest this is an issue that has little to do with the colour or type of paint on your car. Bare metal if you want to be very technical starts its rusting process pretty much as soon as its exposed. Car manufacturers use all sorts of products and techniques to control this and ensure there cars don't have rust issues but this process can and does at times (although these days its rear) fail and the end result is that eventually the end user gets rust spots coming through their paintwork. You can put the best paint in the world over a poorly prepped surface and eventually you will have issues.
I hope that helps.