I think we've hit most of the important points here but from one retreded tanker's perspective,"tank" is just the solution to a continuing need for mobility and protection and firepower in one platform.
Mobility: tracks still generate lower ground pressure for a given vehicle weight than tires (even with central inflation control).
Protection: all of the things that heavy armor is intended to protect a soldier from are still out there (cheap, massed produced, bullets, shells and rockets). Light armor is just to allow you to get away, if you have to take some land away from someone, get the heaviest armor you can.
Firepower: a high-velocity cannon can be used against just about anything except high-speed aircraft just by loading a different round of ammo. No missile has that kind of flexibility.
The M1 bears no resemblence to the WWI British "Male" and armor in the next century may not look anything like the M1 (Ask David Drake, he says in the front of his books he's just trying to tell a good story).
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