Touchscreens came out on the 70D and you can bet that the 6D2, 7D2, 5D4, and 1DX2 will have it.
Minor correction. The 650D/T4i was Canon's first DSLR with touchscreen. Whether the 5D4 and 1DX2 will be equipped with touchscreens depends on performance of those screens under weather testing.
My opening line was "for clarity, I will ignore Rebels, EOS-M, and SL-1 and concentrate on the higher end cameras"
Weatherproofing of a touchscreen is the same as for a "normal" screen. Even articulated screens can be easily weathersealed.
The convenience of touch controls will be hard to resist. Tap the screen to select an object to focus on... double tap to select an area to meter on.... longer tap to select white balance, etc etc.. this coupled with the existing buttons and dials makes for a killer interface.
The problem with weatehrsealing an articulating screen isn't whether it can or cannot BE weathersealed. The problems is that you have to seal a joint, which means wear on the seal, which means it had the potential to break down and break at some point. Worse, you generally wouldn't know that the seal was broken, which actually greatly increases the chance that you might ruin your camera by using it in the kind of extreme conditions that you would never use a non-weathersealed camera.
There is a very specific reason why the 1D X, which has the best weathersealing of any Canon DSLR, does not use a main rotatable dial and opts for buttons instead. The wear on a seal with a rotatable dial is greater than wear on a seal with depressable buttons.
You don't seal the joints.... you seal after the joints so the seal is somewhere that does not move and wear out...
Also, nobody ever said that if the 1DX got a touchscreen that it would be a tilt-swivel screen. My bet is that it gets a touchscreen and it is fixed in place like the current one.
You can't do that, though. The joint is what contains the electronic ribbons that transfer data from the main board to the LCD screen. You have to seal the ENTIRE thing...you can't slip the ribbon through some unsealed slot, that would effectively nullify all of the sealing.
There is no articulating screen with the same level of weather sealing as the 5D III and 1D X currently have. It just isn't going to happen. Not to mention the overall fragility of an articulating screen in the kinds of extreme situations that those cameras can be used in. I've hauled my 7D through thick brush and brambles, dropped it in the mud, had it even topple over onto hard ice on my ultra light weight Gitzo tripod because of 60mph high winds up in the mountains during winter a couple years ago. If I'd had an articulating screen, my 7D would have gone bust a LOOONG time ago. I just walked in the door about 10 minutes ago...at the moment, my 7D is once again covered in mud. It's just the name of the game...if you want to get anything remotely resembling a true professional bird or wildlife shot (and I'm not saying I've ever achieved that goal yet), you just plain and simply have to crawl through the thickets and mid and get really dirty.
"Articulating screens" and "rugged" or "weather sealed" can't be placed in the same sentence when it isn't pure irony. I understand all the arguments for them in high end models, but I think anyone who actually buys one of Canon's xD models where part of their decision is based on the ruggedness and weather sealing of the body, if they had a choice, they would NEVER choose the option to have an articulating screen.