I've boarded / deplaned at the rear quite a few times over the years...but almost always when the plane has no jetbridge and the front is also using stairs.

I can't recall a case where the first X rows used a jetbridge and the remaining Y rows had to walk down a flight of stairs, across the tarmac, and up a flight of stairs to the plane.

It seems to me a net benefit when stairs are already involved and you're just saying "rows 15+, use those stairs instead of these stairs", but not a net benefit when it would create two wholly separate boarding processes.