A new design would only further increase interest in next year’s Apple silicon iMacs, imo. I doubt Apple would would consider a redesign to be a negative with respect to marketing.Doubtful if iMac would get a new body in 2021.
Apple wants you to be focused on the Apple Silicon chip and not the new body.
Separating the redesigned body with the change of chip allows for better marketing for 2021 and 2022 or later Macs.
From November to June 2021 - Apple Chips
From 2H 2021-onwards - redesigned bodies
If I were iMac product manager, I would definitely want new Apple silicon Macs to be wrapped in new metal, anyway.
They could already introduce a lower end 24” iMac with the M1 at any time, and obviously that would be a new design.