It's not just that. China's deterrence requirement grew along with the country. In the past, China was essentially a giant North Korea - too poor and weak to be worth destroying if there's any chance of retaliation. In those circumstances a nuclear policy like North Korea's - minimal deterrence - was sufficient. Now that calculus is no longer valid; China is very much worth destroying, and an American president might decide that it's worth losing two or three American cities if it means being rid of China forever. China is flirting with catastrophe if it doesn't massively expand the quantity and capability of its nuclear arsenal.
The Type 09-IV as it stands is, quite frankly, a joke. Draw a 7000 km circle centred at Los Angeles to see how far into the Pacific that submarine would have to venture to launch. It's even funnier if the target is Washington - there the submarine would have to actually make it past Hawaii. It might as well not even exist, it's so useless. An 11,000 - 12,000 km single warhead (or 2-3 MIRV) JL-2A would enable the 09-IV to hit LA from the northern tip of the SCS, which would have to tie down some American ASW capability; so there's always a reason to do this retrofit no matter what China intends with the Type 09-VI.
I think it makes the most sense for production of the 09-IV to be halted and all SSBN resources dedicated to 09-VI and upcoming designs. The main benefit of a Type 09-VI that can field 14,000+ km 10+ MIRV missiles isn't that it makes a SCS bastion strategy viable; it's that once China establishes naval bases on Taiwan and has a straight shot into the Pacific, the potential launch areas that cover the entire US are staggeringly vast. Only in the Indian Ocean would the submarine be out of range.
