Just because there are no magnum open tops isn't proof it couldn't be done. It only says no one has built one. Why? Who knows? Saying it hasn't been done BECAUSE IT'S NOT POSSIBLE or because IT'S A WEAK DESIGN, until the testing data or engineer's reports are displayed, is just conjecture. Idle opinions.
Just because there are no open tops (currently, or ever) made for magnum loads isn't proof the open top is weaker. The last of the open tops went away in 1873 with the military contract for the SAA Colt. Saying the military didn't adopt the open top due to it being a weaker design, isn't something I've read. As far as I know, that's just conjecture 'everyone knows'. Perhaps the top strap model was chosen because of the ease of cylinder removal. No wedge to lose. Easier for the soldiers to learn and care for. Perhaps the top strap SAA Colt was cheaper to produce so cost less? Maybe it just 'looked right' to the board. Get us the data that supports your view.
Did you know that in the time of the Wright brothers and their flying invention, there were seven other means of heavier than air flight? (don't ask me what they were, that's all I know) but we've latched onto two. Does that mean that all other means of flight aren't possible?
On the other hand, 45D has proven the open tops can handle greater pressures than comparable top straps. Seems proof enough to me. Yet the goal posts get moved out there farther.
You know, I get the idea if 45D built a magnum open top, it would be disqualified for some reason - too big, too heavy, as massive as a Blackhawk, too expensive... it's not possible to win this argument, even with irrefutable proof.