Right now, we don't know what the product will look like and if it will be developed at all. But if its quality (either optical or build quality) falls short of that of the 17-55, I'm definitely not gonna buy it.
Fair enough, we all have different wants and needs. My personal perspective is that a zoom should offer flexibility at "good enough" quality over going for a prime.
Even then we need to define what aspects and weighting we apply, such as but not limited to resolution (centre and border), at various apertures, at various focal lengths and even focus distances. Throw in considerations for vignetting, astigmatism, coma, bokeh characteristic and colour rendition. All in, "better" will be subjective.
If they make such a lens, increasing zoom range may impact some of those parameters. How it does so will be the question.
If this lens does ever get released, you can bet your life it will be 30% more expensive than the EF-S 17-55 f/2.8.
You can see it like bodies too. The new version will always come in at a high price. Over its life it might creep down slowly until it seems a bargain, when they replace it with another expensive one. Canon are not alone in this...