Problem is, the PL-21 isn't in service yet. The R-37M is a nice back-up system in the event that the PL-21 or whatever PL-XVR missile takes longer to mature than expected.
Just saying, while the Su-57 is a disappointment to an extent (it's basically F-22 and F-35s for broke people), the Russian missile development complex is still impressive. A variation of the Kinzhal adapted for the Su-57 could see service on the Su-35, when the direction the Chinese are going in is a DF-21 adapted for the H-6. Likely, the Kinzhal variants will have far inferior range to an air-launched DF-21, but can be mounted on smaller platforms.
You have to remember, there's holes in Chinese missile arsenals. There's the PL-10, but there's nothing known to be comparable to the American micromissile suite, and while there's the PL-21, it's more vaporware than the R-37M.
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Put another way, is it a strength or a weakness that American fighters don't run Meteor? The Meteor missile has incredible capability, especilaly when compared to AIM-120Ds, given its ramjet power, but the Americans don't use it. Why? Because it's not an American missile and the American MIC will scream.
China should not have the same problem. If a foreign system can do a job better than a domestic system, and there's domestic redundancies in the event of embargoes or sabotage, it should use the foreign system with the domestic system as a back-up.