Even if the quality is equivalent the new 24/28/35mm a price 5 times higher than the current 50 F1.8 would scare potential buyers and push them to Sigma.
Regardless of price, an IS lens can't push someone to buy a non IS lens.
These lenses are specifically made with IS for hybrid shooters/videographers. Non IS lenses are not an alternative.
I love IS for a 50mm lens. I want to be able to shoot to at least 1/30 and lower shutterspeed with this focal length handheld.
My 50/2.5 lives on my 6D. If it could freeze water hand held, it would be more versatile. I can usually manage 1/30 handheld, but IS would comfortably allow 1/15 or 1/8. With full time MF override, quiet AF and weatherproofing, it would be perfect. I'm hoping for the new lens to have closer focus than the 50/1.8, 50/1.4 & 50/1.2L. It may well do, because Canon seem to be phasing out lenses with the old noisy focus motor, in favour of USM on full frame and STM on EF-S. Fortunately, the roll out of STM lenses is solving this.
I'm guessing that the new 50 IS will come in at a higher price, and the 50/1.4 will become the bargain model. Then, there is also the 40/2.8 STM to prop up the range, as currently the 2nd cheapest and 2nd lightest prime in the lineup. Canon have to improve the perceived quality at the low end, the focus rings on some of the kit zooms feel loose and ready to fall off when tried in the shop. That might help encourage sales of L-series lenses, but it must also help the sales of mirrorless cameras by Sony, Fuji, Olympus & Panasonic