There's no good reason not to. Enemy (any) fighter jets can only carry so many air-to-air missiles, so quantity of armed platforms in the air has a quality all of its own. Especially if the J-10Cs benefits from scales of economy, and can be put into the air at low costs and high numbers that overwhelm any enemies. Just as an example in a hypothetical scenario, if 100 J-10s with 10 J-20s are being sent to take out the Guam bases during a broader war (while the rest of the J-20s are defending the mainland/attacking more important targets), that will divert F-35s/F-22s from other missions, and they may still end up losing the bases in Guam regardless. The J-20 is obviously superior 1 for 1, but you still need bulk behind it to translate this into winning the air component of a war. J-10Cs are providing that bulk until such time that J-20s can be produced in large enough numbers and low enough costs to support an even better platform (6th gen fighter, etc.).
My question is mainly regarding the report by IISS that China is still somehow building J-10Cs in large numbers in the 2020-2022 period, despite we know that the J-10 production has generally slowed down and possibly already stopped in that period. For that,
@Jiang ZeminFanboy has already answered my question.
However, regarding your points - the J-10C production has pretty much stopped altogether by 2022, as far as we know.
Besides, the J-10s, being a mediumweight fighter, do not have sufficient combat range to reach Guam. That means either the J-10s have to conduct mid-air refueling en-route to Guam, and refuel again on the return trip - Or they would have to land in Guam after Guam has been successfully captured, adequetely resupplied and defended by the PLA.
Therefore, J-10s won't be used for attacking targets in the vicinity of Guam unless absolutely necessary. They are more suitable fighting around the vicinity of China's territorial borders, coastlines and theater of war within and around the FIC.
Furthermore, there is another key reason why J-10s aren't being built in large quantities anymore (or, stopped being built altogether) - Major air forces are moving on from 4th-gen to 5th-gen fighters whenever possible.
The J-16 is still being mass produced because of their excellent utility as a bomb/missile truck for the PLAAF. Any further production of the J-10 series is going to be export-oriented and replenishment for airframe losses in the PLAAF only.
Once the J-35/31 production lines are spooling up, I think we could see J-16‘s production winding down in the coming years. Considering that there would be 4 variants of the J-35/31 according to the chief designer in Shenyang, it is highly probable that the PLAAF would be looking towards phasing out earlier J-10s for land-based J-35/31s for the mediumweight fighter role starting from the mid/late-2020s.
In fact, the the PLA CMC desires, Shenyang could very well start building J-35/31s for the PLAAF as soon as possible. Introducing J-35/31 into PLAAF service at the same time as the J-20 is one surefire way to rapidly expand the 5th-generation fighter fleet in the PLAAF.
The same goes for the US who is still producing F-15EX and F-16s today. Although, unlike PLAAF's J-16, the USAF only intends to procure no more than 150 F-15EXs.