Even 1000 km range is too short for a non optimized plane for the PLA IMO. A F-22 already in the air on patrol outside radar range at 1000 km can detect and close the gap with a high RCS cargo plane at afterburner Mach 2 within 20 min, and the cargo plane would have no way to escape or fight back the way a fighter could. It's very hard for the cargo plane to escape a few AIM-120s while a real bomber or fighter can maneuver away, be not detected in the first place, or deploy countermeasures.
The 1000-kilometer range quoted in my post is based on the LRASM and JASSM missiles used in the Rapid Dragon missile system.
China already has cruise missiles that can reach targets beyond that distance -
DH/CJ-10K/20 can reach 1500-2000 kilometers.
DF/CJ-100 can reach 2000-3000 kilometers.
HN-3 can reach 3000 kilometers.
HN-2000, although having little available information, is said to have a range of 4000 kilometers.
On one hand, much of the fixed i.e. land-based targets which China is expected to encounter in case of war in the WestPac are within 2000 kilometers of the Chinese coastlines and the eastern borders, which mostly doesn't require the use of Rapid Dragon-type missile system.
On the other hand, Rapid Dragon-style missile system grants more flexibility in terms of deployment. They can be used for rapid deployment to locations with poor ground-based transportation (such as the Himalayan frontier) to launch missiles, and/or to launch missiles against time-sensitive targets. Sure, fighters and bombers can do the job better than airlifters, but the airlifter-based "temporary bombers" can either complement these proper units when required, or to fill the gaps needed for a rapid strike when proper units aren't available in the region at the time.
Furthermore, in order to utilize these kinds of airlifter-based "temporary transports", these airlifters would mainly operate within uncontested airspace i.e. territorial airspace and/or airspaces that have been secured by allied fighters and SAMs. I don't see how enemy fighters would dare to breach deep into territorial airspaces over the Chinese mainland just to shoot at Chinese airlifters without successfully disabling much of the area-denial platforms (which is already a gigantic task), so there's that.
Unlike for the US which has a huge domestic civil aviation industry, cargo planes are very expensive in China, more than fighters, and more limited in numbers.
Airlifters are military-focused, not civilian-focused.
Besides, expensive or not, having a large fleet of airlifters is an absolute prerequisite for the PLAAF to become a world-class air force. Plus, China has the economies of scale, meaning that they can build things at a cheaper price and at faster speed than their American counterparts.
Moreover, while the US has stopped the production of C-5 and C-17 completely (the C-130J is still being produced), China is working hard to pump out the Y-9 (and once the WS-20 is ready, the Y-20 too) like no tomorrow.