Better late than never, I think. Unfortunately, ten years ago China's helicopter industry was in pretty terrible shape. I think they've solved a lot of the issues for at least Z-18 production, like engines. They just need to spam them out now.
Z-18 will be essential for Army Aviation to suppelement Mi-17s and replace older Z-8s. The Z-18 also looks like a helicopter that will be great in the CSAR role once the PLA decides to commit to it.
But I think where Z-18 will truly shine is in naval aviation.
Z-18F will offer to PLAN one of the world's heaviest ASW helicopters, that they can field on aircraft carriers, LHAs, and large surface combatants. It will prove a massive leap forward compared to Z-9Cs and Ka-28. It is probably too much to expect Z-18F to provide the standard PLAN ASW helicopter, but it will almost definitely still be a successful product.
I also expect a navalized version of the Z-18A, as the PLAN's standard transport and air assault, utility helicopter. It might not be as flashy as MV-22, but it's got the legs and the muscle for a variety of missions. In future, hopefully we will see PLAN LPDs and LHAs loaded with navalized Z-18As as standard.
But it's probably unrealistic to expect Z-18s to fill every role in the PLAN, especially for ships with smaller hangars. I think that is where Z-20 comes in.
Assuming Z-20 has similar dimensions to H-60, I assume a navalized version should be similar in footprint to SH-60. We know the dimensions of a folded Ka-28 and Z-9C, which a folded SH-60 can fit in. So if a PLAN ship has a hangar big enough for a Ka-28 or Z-9, then it can probably fit a folded Z-20 as well.
This is important for the PLAN's older and smaller ships with smaller hangars (in other words, virtually every surface combatant prior to 055), because Z-20 is basically their only hope for a modern, medium weight ASW helicopter. Z-9Cs and Ka-28s are virtually already obsolete.
I expect a standard transport/utility version of Z-20 as well, akin to MH-60S, as a more nimble complement to the larger, naval Z-18A.
The fact is, production rates for Z-18 and Z-20 alone will probably not satisfy all the demands of army aviation, naval aviation, and air force, but if they produce both at the same time, operated in a high-low manner among all the services -- especially navy -- then they'll have a more flexible force to mix and match as they see fit.