It's true that mirrorless cameras aren't pocketable with a supertele, but then no camera is. 500gm for the 100-300mm panasonic (200-600mm equiv) doesn't seem too unreasonable. Canon's lenses in that focal length range aren't even hand holdable.
Hence, there is a point, in terms of focal length, where the weight of the body becomes irrelevant. Many of Canon's tele and all Canon super-tele lenses are heavier than a DSLR body. Even the 85mm f/1.2 is roughly 25% heavier than a 5DmkII. Never mind a lens like a 400 f/2.8. When working with that kind of glass, trying to save weight on the camera body does not make sense, as it will do little to the overall weight of your equipment.
Thus, if my premise above holds, it probably only makes sense to try to produce quality glass to somewhere between 135mm and 200mm for a lightweight camera.
If Canon could thus build a camera with an APS-C sensor and a 15-85mm f/2-f/4 lens, I think that would cover 80%-90% of the potential applications for a lightweight camera. I know there would be cases where I would be limited by such a camera, but on the other hand, such a camera would satisfy my needs 90% of the time.
Would a mirrorless, interchangeable lens camera be nice? Absolutely. Would I think a lot longer about taking the plunge to a second system (in addition to EOS/EF)? Probably.
The flipside to this is how good a job the marketing departments of the manufacturers of CSCs do.
What I do know, however, is that I want a compact camera with a bigger sensor - preferably APS-C. I am less dogmatic about the need for interchangeable lenses. I am still waiting for something that really grabs my attention.