Airplanes can become unstable with too much weight in the rear and not enough in the front. First class sections carry less passenger weight per linear foot than coach sections.
Wouldn't airplanes be safer if airlines located business and first classes at the rear of the aircraft rather than at the front? To preserve boarding and exit convenience for premium passengers, the aircraft could be boarded from the rear door rather than the front.
How are you proposing to board an aircraft from the rear door when at a gate? This only works when at a remote stand. The impracticality of pulling a plane up to the gate butt first was just discussed on another thread.