Stress on the frame from cycling is an entirely different event than a high-pressure gas release where you don't want it. No comparison between the two events at all.
As far as polyagonal rifling being more prone to leading.... I call hogwash. Why would it? Do the bullets fit? Do Glock shooters slug their barrels, and measure throat dimensions? We discuss leading issues all the time on this forum, nobody says, "Well, your brand X revolver leads, best have a smith unscrew the barrel and put in a different one." No, we check dimensions, see if there are constrictions or rough spots, lap or polish them, make sure our bullets fit the bore, etc., etc., etc.
I'd like to hear from some Glock owners who have slugged barrels as to what dimensions they have. I did not and for other reasons don't have a Glock right now. What about the chamber dimensions, up by the throat? Oversized? Sharp edges? We know those things contribute to leading.
What about firing out of battery? Easy test, prime 50 pieces of brass (no powder, no bullet). Head to the range. Gently clamp gun in a vise or fixture. Chamber a primed piece of brass. Take out an auto feeler gage set, and gently slip the feeler gage in between the barrel shroud and breechface (but not far enough to let the firing pin hit it). Will it fire? If so, go to the next larger. Find out how far out of battery it will fire. Before you say it won't, there must be some tolerance, or the least bit of crud would put it out of commission. OK, find the range where it will operate. Now, to be scientific, try the same experiment with your favorite other type of autoloader (ie 1911, sig, smith, taurus, ruger, HK, et al).
To blindly proclaim it's a rifling issue is to ignore a great many other factors that are relevant. Why we ignore these factors with a glock and not other makes of firearms is beyond me.