If they could stretch the expected KY-600(?) and de-navalise it to reduce empty weight, it could IMO be a relatively simple and cheap fix to replace aging Y-7s with the added bonus of good takeoff performance due to originally being a carrier transport. If Y-30 is going to be a Y-9 type transport with ~25 ton of payload I think it could be pretty overkill for Y-7's mission set. Considering PLA has not outright retired Y-7s and to my knowledge kept a decent amount of them in serivce probably means they still see purpose for a Y-7 class transport.
The PLA probably do think that the Y-8/9 (and the upcoming Y-30) family is pretty sufficient for conducting the same mission sets as the Y-7, whilst being more capable than the Y-7. TL; DR - The Y-8/9/30 can do everything that the Y-7 does, and then some more.
Hence reason for keeping the Y-7 in service could be to keep the airlifting capacity filled while waiting for newer Y-9 units and the currently-WIP Y-30 to be rolled out.
Going forward, keeping only the Y-8/9/30 category instead of both the Y-8/9/30 and Y-7 categories of airlifters likely also help a lot in terms of simplifying and streamlining procurement, deployment, logistics and maintenance efforts.
