I'm a manual trans fan, but my Ranger is an auto simply because that's what I found in good shape when I was looking. When I drive an automatic I just drive it as it was designed to be used, not as I would would drive a manual. It works fine actually.
When you brake using any method, you are transferring kinetic energy into heat which goes into the air around the vehicle. If you use the brakes, then the kinetic energy of the vehicle goes into the pads/shoes/rotors/drums, and from there into the air - it's a pretty short path and the components were specifically designed to do that. Also there are brakes on all 4 wheels so the load is shared.
When you engine brake with a manual the kinetic energy of the vehicle is transferred through the drive gears and transmission to the engine, where it's turned into heat through compression and flows out the radiator. A much longer path, but the gears transfer energy with very little loss, and the engine and cooling system can handle it. It saves on brake lining wear, so it's useful.
With an automatic it's similar, but there are more losses in the auto, so the transmission will take some of the heat load (depending on if the converter is locked, etc.). The rest will go out the radiator as with a manual.
If you think about it, why do you want to send that energy through the drivetrain, especially the automatic trans? Why not use the brakes - they were designed for that and are much more easily (and cheaply) replaced? It sucks that it's not a manual, but in fact it's not a manual and no amount of pretend shifter games will change that.