On the contrary I think a B757/767 sized plane would be too small of a "jump" or offering from the C919. As it currently stands a three-class seating on the CR929-600 is aimed at 260-280 seats, which puts it in the B789/A359 range - so not as big as a you'd think, definitely not B77W/77X or A35K sized. I'd say the current plan serves the right amount of "plane" to provide the long-range needs of major airlines in China and Russia, alongside the smaller operators.I believe size choice was/is also flawed. It would have been better for commercial sales if middle of market sized plane was the goal. So something in between b757 and b767.
It'd be more sensible to offer an A321 sized C919 variant to address the B757 (180-230 seats) need in the industry once the C919 program matures and engineers get enough experience. Similarly a shorter CR929 along the size of a B767/B787 (220-250 seats) variant can be offered using that line of thinking.
Just my two cents as a commercial pilot anyway