Drive into shallow water leveled just over your flat pipe and stop the engine. You will be surprised.
I can't even figure out which pipe you are advocating for anymore. Which one are you calling the flat pipe? And which one would you prefer?
I realise you're the expert on all things but you know, I probably wouldn't be all that surprised. In my experience (grew up in Aus outback) drowning the exhaust isn't causing problems unless the water's so deep it can get to the exhaust port. Otherwise the pipe just floods, and drains out when the machine is recovered, or the engine starts. If it were such a problem, my ski boat engine with wet exhaust would be destroyed every time the engine stops. The exhausts are completely submerged apart from a short rise to the manifold. No airlock. Just a rubber flap to stop other boat's wake going up there. Muffler baffles will do the same job.
What exactly are you getting at?