If you actually look at the international market, airlines are moving away from regional jets sized airliners and onto larger airliners. The smallest jets that airlines are now ordering will probably be CS100 or E2 size going forward. Which particular routes in China do you think need something of ARJ-21 size that something like CS100 can't do for greater profit? Remember with expansion in size of air travel, the number of runways and airport slots are going to be more restricted going forward. So airliners are going to use larger planes because they can fill them. Most of the LCCs are ordering planes that can sit from 180 to 240 people. You simply cannot get the same CASM with regional airliners as A321 if an airline can fill all the seats. There is really not a lot of point to stretching ARJ-21, since the need for that really isn't very high anymore. How many airlines are looking for aircraft with 110 seats in single class config. If you stretch ARJ-21 to be able to seat more than 110 people in single class config, you get into the territory of a shrunk version of C919.
As for profit level, every new airliner program looses money in the beginning which it recoups later as it lowers the cost in its mass production. So unless ARJ-21 can sell substantially more, it's going to loose money. I don't think that's a huge problem, since the main focus here should be C919. It's aimed at a much larger market. It's a far more advanced project than ARJ-21. The greatest value of ARJ-21 is the experience that it brings to COMAC in future projects like C919 and C929.
The long delay to bring ARJ-21 to the market has caused it to miss its best window of opportunity in China, what with the high-speed rail network spreading like a wildfire now.
That said, there is still a sizable market and need for a regional jet of ARJ-21's class in Southwest and Northwest China, where the geography and terrain make it difficult and costly to build extensive rail network. As yet, those parts of the country are not wealthy enough, but they all have growth potentials. Many of those places are or will be attractive tourist destinations as the facilities and transportation improve. ARJ-21 would be a good choice linking the smaller cities in those regions to the major cities of the country, as the spokes in a hub-and-spoke system, or between them in a point-to-point network. Over time, there will be demands internationally, more likely in developing countries, as it builds trust and reputation. COMAC is also planning to modify it for the business jet market.
We should not judge the success of ARJ-21 in isolation. A lot of the investments into the project are in reality investment to build an industry base and infrastructure for the entire civilian aerospace industry in China. It's a down payment for the industry. To apply the criteria that Mr. Luo Ronghuai uses, from the interview that Hendrik_2000 posted earlier, if ARJ-21 achieves the technical success and market success, it should be considered a success in itself. If the Chinese civilian aerospace industry takes off, represented by the success of C919 and the following models, then ARJ-21 should be considered a complete success. Commercial success would be the icing on the cake in this case.