"Patrol duties" could be and now are easily handled by 056s. In fact aside from antiship missiles a 051 is inferior in every other aspect of combat compared to a 056 despite having almost triple the displacement. I suppose you could keep the Luda IVs around to serve as antishipping missile trucks, or 4 fighters could do the same job faster, with greater flexibility, and at lower cost.
As for personnel experience, I think there is a usefulness to having a large pool of experienced sailors. On the other hand, the kind of experience that you gain on a 051 could only transfer in the most general ways to a more modern vessel, like basic seamanship, leadership skills, and what not.
I agree that the need for older destroyers for low intensity patrol duties is declining as more 056s enter service in large numbers. Though I think there is an argument that could be made that the larger size of some older destroyers despite their older capabilities will still allow them to remain at sea longer than 056s -- there are some specific, unique roles where that could suit them which an 056 may not.
That said, I think the more primary reason for retaining them for a little longer is to maintain the fleet structure and experienced personnel that goes with them. As more 052Ds are commissioned in the next couple of years, keeping these last few 051s around will probably allow that transition to occur more smoothly.
This year will see at least two, maybe three 052Ds commissioned, and next year we may see three or even for commissioned. Along with that I expect to see the last five or so 051s to be decommissioned by about the end of next year as well, by which point the oldest destroyers in the Chinese Navy will be the pair of original 052s.
I am exactly suggesting to phase out all ships below a certain level of technological capability. By way of an example, the USN retired all five initial Ticos that used the Mk 26 twin-arm launchers instead of VLS because they were deemed unable to cope with a Soviet saturation attack, and this was before they reached their full useful hull lives. The Thomas S. Gates was only in service for 18 years before she was decommissioned.
I think there is reason to ask whether the USN's retirement decisions for early Ticos (and indeed their Spruances as well) was a good decision in the long run.
That said, I agree that in time the Chinese Navy should standardize so their assets are at or above a minimum level of technological capability. I just don't think it will be in the immediate future.
I forgot that the 053H3s have recently been refit with HHQ-10s, modern decoy launchers, and 2 730s (though with a single FCR). They are now essentially a larger 056 with hangar capability. Adding VDS/TAS would making them a pretty formidable ASW frigate. But seriously, the Ludas and the Luhus just have to go. IMO they are more a liability than an asset at this point.
The old 051 Ludas will definitely be gone by the end of the decade (probably within the next two years) and the two original 052s will probably be retired by mid 2020s. Until then, I think they, along with other older destroyers will still serve for personnel retainment and ease of organization, as well as operating as longer endurance low intensity patrol ships.
I'm not sure what's going to happen with the two 052Bs and two 051Cs, but I wouldn't be surprised if they're refitted with the HHQ-16 weapons suite like the 051B and Sovs are... so by the mid 2020s I expect the Navy will standardize to an 054A level of minimum technological capability, at least for destroyers.
For frigates, I expect many pre 054/A frigates will be modified where HHQ-7s are replaced with HHQ-10s and other minor modernizations will allow them to use for general patrol/ASW patrol role to supplement 056/As.