Very few prints receive massive amounts of direct sunlight at close range. I'm not sure where you are getting your 50c assumption but not buying it.
There's no 'close range' when it comes to sunlight. It all comes from the same source 150 million kilometres away. If an object is receiving direct sunlight, it makes no difference whether it's 1cm away from the glass or 10m behind it. A print on a north-facing wall 3m away from the window receives just as much sunlight and gets just as hot as the same print right up against the window - just that the print on the window will be irradiated for longer than the print on the wall (due to the movement of the sun's relative position in the sky).
It's fairly logical, really. If you put a print inside a frame, you've created a close environment with minimal air circulation. Dark objects (e.g. prints) absorb much more radiation, and become much hotter than, light-coloured objects. This heat has to go somewhere. The print absorbs heat from the sun and, in turn, heats up the air in the frame. This air is trapped there and has nowhere to go - it just gets hotter and hotter.
If you don't believe me, try sitting in a small room with lots of sunlight on a hot day, with the door closed. Then do the same thing in a large room with lots of sunlight and the doors open for air circulation. I can guarantee that the small room will heat up a lot faster than the large room.
I don't know how you measured the temperature of your prints but I wonder if it was scientific enough to really know how hot the actual paper was. Measuring the air around the print isn't the same, and even 40c is pretty excessive. Certainly while short periods of time are not a problem, extended exposure to heat is. I just don't see any correlation to real world situations.
Infrared thermometer. The same one I use for measuring body temperatures of patients, as well as measuring the temperature of heated metal. It doesn't measure air temperature - it measure temperature through the blackbody radiation of the surface.
I've done many fading tests like this over the years ... back window of my car, window of my shop, and any circumstance which allows enough sunlight to fade images quickly also subject the print to temperature extremes. The prints always curl or sort of fall apart ... something I don't see with many decade old prints that haven't been subjected to the same treatment.
The amount of solar radiation you get in 3 months over the spring/summer is equivalent to a lot more than a few decades of typical use.