J20's scale up production has already resulted in shut down of J11 production line due to functional overlap.
It's just a no brainer that the same will happen to J16 once J20S is ready to be commisioned.
As others pointed out here, J-16 serves a different role compared to J-20. The advantages of J-16 are,
1. It is cheaper to operate. There is no need to apply RAM coating etc. and to house it at special hangers.
2. It carries far more payload due to not needing to be stealthy.
3. It is multi-role, having two people onboard will help with ground attack operations. While there is already a two seater variant for the J-20, at least thus far, the Chinese have not made all of their J-20 two seaters.
4. Available production capability means the Chinese can increase the size of the fleet much faster if both are produced. You can only have so much production capability for a given plane. Once you invested the cost of the production line, you will want to make best use of it while what you produce is still relevant. This even more true if you consider the initial limitation of the WS-15 when it becomes available. It would mostly be allocated to J-20 and there would be a limitation of how many are produced.
5. There are other use of the plane where the J-16 is more than good enough to do the job. The Sino-Indian border is one such case.
Even the U.S. with our fleet of F-22 and F-35, are still producing F-15s for our military today.