The examples you quote above have the advantage that commonality is, or has been, an objective when one, if not both, types are/were at the design stage.
That's nothing new, of course, it's almost 40 years ago that we were first getting excited about the 757 and 767 having a common type rating.
But here we're talking about two independently developed products from different manufacturers, both already certificated and in service. Trying to reverse-engineer any meaningful commonality between the current Airbus narrow-body family and the C Series would be both hugely expensive and ultimately have very little point to it. Not what most would describe as a "logical step".