They grandfathered the frame--and added MCAS-- to avoid a new certificate and added cost/time to their customers. What you point out is one of many consequences

They grandfathered the frame--and added MCAS-- to avoid a new certificate and added cost/time to their customers. What you point out is one of many consequences, not the reason. (Aircraft doors are interesting on their own as well, due to the assumptions that drive design, testing, and certification.)