This might not be a question which is capable of an answer, but why aren't all cameras designed so as to be able to use a wide variety of flashes, including vintage flashes? I'm no engineer, but to me a camera that can be "fried" by the wrong synch voltage has been very badly designed. Why don't the problem cameras include something like the electronics in those add-on safe synch products?
Maybe its because the camera manufacturers want to sell their own flash guns at exorbitant prices?
My guess and this is a guess.
Most modern flashes have a similar safe sync built-in.
Modern cameras use solid state switching circuit and not an electro mechanical switch like old cameras. These devices would last much longer than the mechnical type especially with repeated use. These solid stated devices can only work with a certain range of voltage and not almost all like a mechanical switches. If they have to limit to a range of voltage it's better to use the lower one as it's safer. Inside a flash unit high voltage switching is still needed but a device to switch high voltage circuit with low voltage (that is what the safe sync does) can be built inside the flash and there is no exposed high voltage on the outside of the flash.