Z-20 or to be more exact, Chinese version of the S-70, has been around for a long time, so until there's an actual unit in flight, I won't hold my breath.
It actually would make sense for PLA to use a 4 ton, 7-8 ton, 10-11 ton and 14-15 ton helicopters at the same time. We're not talking about today or even in five years time, but a more distant future. 4 tons would be z9 or its successor, 7-8 ton would be z15, 10-11 ton can be z20 and 14-15 can be some further variant of z8 or its successor.
Currently PLA uses so many mi17 class helos not because it needs exactly such helicopters, but because there were no other options. z9 was clearly too small for a good deal of uses, z8 had issues and wasn't available in greater numbers until in recent years. mi17 was literally the only other choice on the whole market, domestic and abroad. Hence its disproportionally high numbers. mi17 might have been too much of a helicopter for a good deal of missions, but there was nothing else in between of it and z9.
Of course, now with z15, z20 and further z8 variants looming, future of mi17 seems limited. Sure, many will be serving still within 15-20 years time, simply because there are a lot of recent builds, but as the whole helicopter fleet grows, ratio of mi17 in it will diminish.
The honest truth is that Z-9 is too small and light for practical military ops, other than SAR and recon, but the former can be better performed by bigger birds (larger payload, more loiter time and range) and latter now increasingly UAV domain. That leaves Z-9 better suited for air ambulance or police helicopter kind of work, which its origin, Eurocopter AS365 Dauphin, predominantly used in such roles and capacities.
Now, if the Z-20 project bears fruit that'd leave the Z-15 with a more uncertain future - though its size means the current generation of helicopter-operating surface combatants, with the exception of aircraft carrier and LPDs, only fits for Z-15, that can be amended in next generation's line of DDGs/FFGs, but that might translate the naval variant of the Z-15 a limited fleet with expected service life of 15-20 years (that goes by current fleet's size and expected lifespan); army and air force might pick Z-20
en masse provided it meet the highland operation requirement, which now only the Mi-17-V7, for the sake of streamline its helicopter fleet.
The carrying capacity (volume, not weight wise) in the likes of Mi-17 series and Z-8 means they still holds unique advantage, it'll be a matter of up-engine and avionics to keep them in the game, though their older variants will eventually have to give way to Z-15 or Z-20.
If Z-20 won the lion share in the military market, Z-15 will need to rely on civilian market, a potential gold mine forecast by industry's experts in 10 years time, when the "open sky" roadmap is fulfilled.
Helicopter production will be a growth industry in China in the years to come.
2008's Sichuan earthquake was a rude wake-up call for China, on how inadequate they are in terms of helicopters and heavy airlift capabilities. That's why Y-20 got green light in that year. Helicopters doesn't get as much limelight but with Z-19 and Z-10 now progressing into serial production phase, the R&D capacities should have been transferred towards new helicopters.
