But China does not have a global presence, so does not need even half that many tankers. And an airliner is more suitable for converting to a tanker. So in 10-20 years when China needs significantly more tankers, China will probably have a domestic wide body airliner that can fit that role.
Even without establishing global presence, having a sufficiently large/sizeable refueling tanker fleet is a must requirement for any strategic-type air force (战略型空军), of which the PLAAF is vehemently striving to become one.
Besides, consider this:
China has two massive frontiers (WestPac and Xizang/Tibetan), both of which have upwards of 1500 kilometers of frontier depths to deal with. These two frontiers aren't exactly dotted with plenty of allied airbases for PLAAF and PLANAF warplanes to refuel, meaning that long-distance transit between friendly bases and combat zones are the norm.
And since
on-station duration is a major influencing factor of overall combat performance of air forces for any large-scale aerial warfares (let alone the sheer size and scale of the WestPac frontier, plus the Xizang/Tibetan frontier to a somewhat smaller degree), spending many hours just for travelling back to bases to refuel before heading out for the combat zones is only going to make things worse.
That's why refueling tankers are a critical part of large air forces. They are essentially force multipliers by increasing not just the number of warplanes available in the combat zones, but also increasing their on-station durations by significantly reducing the need for them to return to their bases every time their fuel gauges show "nearly running on fumes".
The above talking points haven't yet include the fact where precious
aircraft fuel and
aircraft engine lifespans would've been saved towards allocation for warplanes to conduct their assigned missions instead of just spent on transiting between combat zones and friendly bases.
As a matter of fact - With 5-6 aircraft carriers in the Red Sea, Arabian Sea and the Persian Gulf, alongside plenty of friendly air bases made available to the Coalition air forces within 1500 kilometers of Iraq, guess how many refueling tankers were involved/deployed to the region for Desert Shield and Desert Storm?
About 300 of them (256x KC-135s and 46x KC-10s) - And that's just for a theater of war
about the size of Gansu Province (for Chinese users) and
slightly bigger than the State of California (for American users).
Now apply that for
the entirety of the 1IC and up to the 2IC, i.e. literally oceanic/continental distances. You see where this is going?
You sound like you/PLA think war is imminent. I'm more used to the old PLA approach of quick small steps. Like 052C is a very capable ship, but China did not mass produce it even though China desperately need such destroyers with good air defense capability at the time.
The analogy is wrong.
The 052C-class DDGs were only built in smaller numbers because they are the testing and verification platforms for a lot of new technologies for China's next-generation (at that time) DDGs, i.e. the 052D-class. And as the 052C-class stumbled upon several factors (the moving of Jiangnan Shipyard from Pudong to Changxing, indigenization efforts of the gas turbine engines, technological maturity factors of the earlier units) which caused an 8-year-long break in their construction, the first 052D units have already become mature enough to begin serial production during that same period of time, whilst being considerably better than their predecessors. That's why only 6x 052Cs were built.
On the other hand, speaking of the usage of YY-20As and Y-20B MRTTs for aerial refueling - The PLAAF and PLANAF already have the HY-6/U/D/DUs as their first ever refueling tankers since the the 1990s and 2000s, which also serves as testing and verification platforms for them to study, develop and refine aerial refueling operations. And by the time the Y-20 platform was getting ready in the mid-2010s - It's only natural for the growing demand for refueling tankers by the PLAAF and PLANAF made them to be based on the Y-20.
Also, unless there is a guarantee that the C929 can be made available for military-based applications within the next 10 years (of which there is none), then the Y-20 is the only optimal platform available to China. Plus, the higher ups of the PLAAF and PLANAF certainly don't have the patience to wait for the C929 to become ready, especially with the current geopolitical development in the WestPac and elsewhere in mind.
Last-but-not-least,
@Tomboy and
@lcloo have explained as well: 不怕一万,只怕万一.