It has been a while, but in 5 years of repaining A series cameras at Canon Australia I may have seen 1 camera that needed the mirror governor mechanism replaced (and I think that was the AE1 that had a very strange fault, where the first exposure after the camera had been sitting for a while produced a photo that was 1 1/2 stops underexposed. We ended up changing almost everything on it. In the end it we found it had a bent chasis, and was fixed with a large hammer and a anvil..)
There was no plastic part I can recall on any of the A series that needed regular replacing, especially a part that could have been replaced by removing just the top or bottom covers..
Actually - the only part I can recall that was a semi - common problem that could be fixed by removing the top or bottom cover only was the release magnet on the bottom of the camera, which could become sticky so your camera wouldn't fire when you pressed the shutter button. To replace one of these you only needed to remove 2 screws - and undo 2 solder contacts....
I used A's professionally to shoot motorsport for a couple of years before I replaced them with NF1's, and never had a problem with them.