There is a need for IS in prime lenses in the normal and wide angle area. The f2.8 24-70L II with an IS would be great for street photography or handheld landscapes or portraits in low light. The current non IS lens is fabulous in picture quality. But combined with a EOS 5DS (R), you need even shorter exposure time than the usual rule 1/focal length. I need around 1/100th second at 24 - 50 mm range and around 1/125th or 1/160th of a second when shooting at 70mm in order to have crisp handheld shots. A modern four stop IS would do marvellous things and make the lens much more versatile. It should be technically possible, rather easily.
I would be prepared to accept 200 or 300 USD more in price and 150 grams more in weight for this added quality in low light handheld situations.
The 24-105mm f4 IS is not an alternative. It´s soft in the corners by default, a significantly poorer optical quality than the 24-70 f2.8 II.
If you think the same, please respond. If you have concerns (feasibility, price etc.), please post here. Thanks.