Interesting thread, with the "usual suspect" responses. Leaving aside the bias you will get by posting this in the M8 sub-forum
the only sensible answer is a camera that has already proved it's longevity over that period of time, which in turn means a film Leica. Although myself a very happy M7 user if I was buying new today I would buy an MP. Secondhand, I already have the camera that you seek - an M2.
Frankly, I would rather bet on film still being available in 20 years than batteries and electronic components...
Regards,
Bill