As far as this lens goes, it is a very smart move by Canon in that it allows DSLR owners to carry a much more compact kit on their walking about forays, it affords newbies low cost entree into the land of the oft vaunted prime lens, and gives Canon breathing room while they finish up their mirrorless offering.Comparing this, with theoretical dimensions, to the 35mm f/2, at most it will save a few oz's and about a 3/4" in lens length. The 35 f/2 is 1.7" long and weighs 7oz, the 50 f/1.8 is 1.5" long and weights 4.5oz, and the 28mm f/2,8 is in between those at 1.6" and 6oz. So Canon already has 3 lenses around that focal length that are light and small. And one that gathers an extra stop of light at a cheaper price.
Always good to have options, but, it's not like this pancake is solving a problem Canon had. A DSLR in general is not "pocketable" like a mirrorless with pancake could be; and no lens will make it that way. And few, if any, of Canon's cheap primes way more than 1/2lb, so weight was never an issue either. Not for a 2+lb DSLR at least