Chaplin was a brilliant filmmaker, his stuff remains watchable to this day - but no one could seriously look at his output and call it anything resembling reality - even at the time.
You're right - movies do not function as a reflection of reality. The Gold Rush is no more a documentary than Nosferatu.
But you can say the same about Michelangelo's David. An artist's output - and preserving it, is about preserving the culture, not a snapshot in time. "What did the produce, and why?" are compelling questions to ask.
Chaplin was a brilliant filmmaker, his stuff remains watchable to this day - but no one could seriously look at his output and call it anything resembling reality - even at the time.