Resources may still be needed for certain buildings (Coal for Factories for example)
Yeah but no doubt unless settling/taking over a new city, you will have built the resource required buildings during the time they were invented when the resource was strategically important.
Perhaps Modern Armor, Helicopters, Jet Fighters, and some other units will end up taking both Oil and Aluminum in the final version of the game. Making Mechanized Infantry resourceless, as you note, is another option (and it seemed the most likely option).
I hope that Paratroopers aren't considered the standard resourceless unit of the Modern Era.
No suggestion so far that any unit in any era requires two of any resource, i.e a Knight simply requires horses not Iron&Horses, while I don't think this is very realistic, it is probably better for game play reasons.
Why do you hope that Paratroopers are not the "resourceless" unit of the Modern Era, to be they seem to be the perfectly suited candidate, they only have a gun aftera;;, they are the most comparable to the last resources unit which would be Infantry I guess. (though they do call on a plane to magically drop them, but so do archers call on whatever strategic resources a boat might use to magically transport them, so its not really a valid arguement, its an ability like embarking, and one can argue no plane exists to transport them, its just magic)