There is no demand because it has been replaced with more modern aircraft like the Airbus A220. But in COMAC's case there is a huge gap in their lineup where there is no 150 passenger aircraft. The ARJ-21 can only carry like 100 passengers while the C919 carries like 190 passengers.
So the market for such an aircraft is clearly there. And given the state of the Chinese airline industry, which is basically state owned, the government can just make them buy the COMAC plane instead of the Airbus A220 or the Embraer E2. It is also much cheaper to make a shortened version of the C919 than develop a whole new aircraft.
Just because there is a gap, doesn't mean there is demand.
for example, United Airlines future fleet plans have a huge gap between 76-seat regional jet to 160 seat MAX8. Why do you think airlines need a MAX7 or A319NEO sized aircraft?
Take a look at Easyjet. It was at one point the largest A319CEO operator. And then decided the economics are garbage, so it's future fleet plan are A320/A321NEO. No A319NEOs
On 15 May 2017, EasyJet announced the conversion of 30 A320neo orders into A321neo aircraft to be used on busier routes.
The then-CEO of EasyJet, Carolyn McCall, stated of the change: "bigger planes would help EasyJet increase capacity in slot-constrained airports at peak times, such as Geneva, Amsterdam and London Gatwick". She added that the A321neos would help to cut costs by 9 per cent.
The company took delivery of the first A321neo on 18 July 2018 at the
I remember this issue was heavily discussed at a.net when it came out.
Economics dictate that an aircraft CASK continues to come down as you stretch it further and add more rows of seats. Demand for A322 would be far more than A319 for example.
In JetBlue's case, it had a replacement program for 100 seat E90. Instead of going with the similar sized A220-100, it directly went to A220-300 and said that the extra 40 seat could just be filled with lower priced ticket.
Stretched C919 will have demand for sure. As COMAC make changes to C919, it's range and efficiency will improve. With such improvements, a stretch C919 will have adequate range for domestic option. Over time, you'd see more demand for stretched vs original and most optimized version. And then you go through a re-engine program. Normally when that happens, the stretched version becomes the most optimized member of the family and then you want to do a second stretch. Over time, the original version will loose its appeal.
You are seeing that right now with A320NEO. Most airlines would rather have A321NEO. You can take a look at the last 5 years of order history in the NEO program