Well currently, the MRLs belong to the PLAGF and their priority will be providing direct fire support/suppression for the grond forces while suppressing the Ryukyu island is more of a PLARF's job. DF-15 has way more varients than most people thought and still they are serving as one of the primary firepower for the potential Taiwan conflict. They are cheap, abundant and very matural to operate just to strick some secendary targets. If there is a need for a new TBM, they may use some modified domestic M20s, they are cheap as well and with gliding tragectory and more importently, can be mass produced in no time. And since PHL-16 has a modular design, it should not be an issue to launch the M20 (or improved/extended range version) with some minor modifications. In fact, delivering firepower is no longer an issue for PLA at this stage, how to perform real-time recon for targeting is the real challenge.DF-15 is getting retired (if not already, or only remained in the training brigades to clear up the remaining stockpiles).
The DF-16 and DF-17 certainly aren't cheap. This is apart from the facts where the DF-17:
1. Can actually strike father away (and potentially reaching some parts of the 2IC), and
2. Has many other more important targets to strike farther away.
In fact, I'd say that the DF-17 (and even the DF-16 to a certain degree) should be reserved for more critical targets along the Ryukyu Island Chain + Taiwan, and let the TBMs + cruise missiles do the heavy work for ground-based bombardment missions.
In addition, there's the Xizang frontier to consider as well.