The reason that S&W felt it necessary to "beef up" the new K-Frame .357 Magnums actually had nothing to do with the strength of the frame itself, but rather with the geometry of the forcing cone. You see, the original K-Frame .357's (all older K-Frames actually) required the outside of the forcing cone to have a flat spot ground into it at the 6:00 position in order to make clearance of the yoke when the cylinder was closed.
This flat spot created a thin area in the forcing cone that, if large amounts of the right (or perhaps wrong) sort of ammunition was shot could occasionally crack. Now, please bear in mind that cracked forcing cones, even in older K-Frame .357's, is a very rare occurrence and that the vast majority of people who mention it have probably never seen it happen in person.
Among the small number of guns which did suffer this problem, nearly all had been shot with large amounts of full-power .357 Magnum ammunition with 125 gr or lighter bullets and many of them were found to have excessive fouling in the forcing cone that was allowed to accumulate due to inadequate cleaning. Also, bear in mind that when K-Frame Magnums were introduced in the 1950's, .357 Magnum ammo with 125 gr and lighter bullets was non-existant commercially so its effect on the design was never considered.
The "beefing up" that S&W has done to the new K-Frame Magnums is really nothing more that a slight tweaking of the geometry of the frame and barrel to eliminate the flat spot on the forcing cone. The J-Frame Magnums never needed this because their forcing cones, while thinner all the way around, did not require the flat spot at the 6:00 position and because their forcing cones do not extend as far into the frame window and are thus better supported by the frame itself.