I may have missed it, but I think the poster was hinting at technical reasons, not regulatory ones (RVSM) you need autopilot near the tropopause.

The obvious one is WWII planes you speak of were straight wing, whereas modern jets are swept-wing, which are extremely vulnerable to dutch roll at high speed/high alt when flown by hand. Autopilot usually incorperates yaw damper, which eliminates or greatly reduces dutch roll with automatic rudder inputs.