Actually, the reason why A320 does not reflect the Sale of Larger or Stretch Version of A320 series is not the same reasson why C919 will not sell.
I am afriad this post will be deleted if I start talking about A320 and why A320neo have more order than A321neo, so instead of talk about it indetail, I will just say this, A320neo are a favourite amongst low cost airline, and these airline have one concern in common, overhead cost.
As for C919, i am not ruling out anything, I simply say the lacking of Frieght Version, Stretch Version of C919 will make this aircraft less competitive than the Boeing and Airbus. To be fair, no one, even COMAC included would have thought they can really challenge the big 2 with C919. In fact, I don't think anyone sane enough will say that. The fact that C919 retain 500 order (half of them are firm) is not something of a milestone, considering COMAC project to sell over 2500 orders domestically and internationally.and most aircraft depends on pre-order to survive, this is not a very good sign for COMAC which offered C919 on sale for almost 7 years now.
Of course they can make Stretch Version or Frieght version in the future, but then wouldn't what I said is ture then? For which the original C919 is just a steepping stone project for soemthing else. just that time time it's the improved version of C919.