I haven't forgotten about J-16, though I see both J-16 and J-11D as China's equivalent to Su-30SM and Su-35S. Russia has plans to upgrade their Su-30SM to SM1 standard for better compatibility with their Su-35S, I'm assuming that J-16 already uses same avionics as J-11D so therefore it can be argued that J-11D isn't needed.
Playing devil's advocate, a rationale for PLAAF acquiring J-11Ds is so that J-16s can be more focused on its strike role while J-11D can be assigned to long-range air-defense among China's outer airspace frontiers like South China Sea or East China Sea, kinda like how the USAF has its F-15E focused mainly on expeditionary strike while F-15C/Ds are assigned to air-patrols within the US. The J-11B itself is no doubt already tasked with the heavy air-defense fighter role within the PLAAF but J-11D not only has better avionics but also has aerial-refueling capability, which J-11B lacks.
Although, it really depends on the choices made by the PLAAF on whether they want to buy the J-11D or not. Since the J-16 itself is perfectly multirole, not only is it a long-range heavy strike plane but also a long-range air-defense interceptor. J-16 would be indeed be a better choice since it features a secondary crewmember, important for air-defense intercept, while J-11D only has one.