To add my 2 cents, the biggest issue the H-6 series faces is the lack of growth potential. For while the PLAAF had made great efforts in prolonging it's service life don't get me wrong. The plane is still a 1950s design that is adapted to fill roles that it is not originally meant for.
In the face of advanced AA systems and fighter planes it is clear that the days of dumb bombs raids or even precision guided ones are a very risky business, so any modern strategic bombers have to make use of ASMs. The H-6 can do that but the number of missiles it can carry is limited compared to the other competitors out there, also externally mounted wing points drastically effects the flight performances of the plane which again was not an issue during the 1950s and thus far we have yet to see any rotary launcher being developed for the H-6.
Well, the original B-52 has evolved to fulfill roles it wasn't originally designed for either.
H-6 obviously carries only a fraction of what B-52 does, but H-6 is also only a fraction of the size and weight of a B-52.
With the H-6K/J/N, it seems like the PLA are quite content to carry stores externally even if it does effect flight performance. Going forwards I see a long future ahead for the H-6K family as new payloads are developed.
It will remain a potent regional strike platform with the ability to haul a larger payload than what any strike fighter in current inventory or foreseeable future inventory can carry. Even without stealth, it will remain a relevant capability for the PLA and its regional and strategic posture.
For true strategic bombing with longer range and larger payloads, H-20 will be needed, because for those kind of distances involved you'll probably want a much more survivable platform anyhow.
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Actually the Tu-16 was originally designed to be a free failing bomb bomber from the start, the missile upgrade was more an afterthought (Badger B variant), with the soviets developing the Tu-22M afterwards which was designed from the ground up to be a missile carrier.
While it is still the only plane in the PLAAF that can carry large sized missiles, it faces strict competition from the likes of the Tu-22M/160 and the B-1 of the US. So I won't put it pass the PLAAF if they are desirous for a newer design that borrows off the H-20 in some ways that is better suited for their A2/AD strategy, a larger bomber can free up more pilots and offers new growth expansion.
But this is a difficult thing to predict, seeing as so few nations are developing strategic bombers it is hard to forsee a trend or strategic thinking. But for the PLAAF, "larger" is certainly one of their priority.
Developing a larger bomber without sufficient survivability for high end missions would be a poor use of money, given the point of having a larger bomber is to travel further and deliver more payload. For the PLA, such a demand would only really be worth developing a brand new aircraft for if it's against a high end foe.
... so I would be very surprised if the PLA sought to develop a heavy strategic bomber that was different to H-20 (i.e.: lacking its survivability features)
If anything H-20 would be quite well suited for the PLA's overall strategic posture as well for long range regional/westpac missions.
Also, the H-6 platform doesn't really face "competition" from the Russian or US bombers, because China isn't able to procure those bombers anyhow.
What the H-6 platform competes with is the rest of the Chinese strike and bomber types, for the mission of conducting regional bombing and strike against opfor targets and environments.
No other Chinese combat aircraft has the payload and range profile of an aircraft with an MTOW of 80 tons and a payload of 9 tons.
Even when H-20 enters service, the operating cost of the H-6K family will likely be far lower than the H-20 and it will remain a useful platform for delivering payloads in lower end conflicts or more permissive environments, as well as carrying oversized unitary payloads which can't fit H-20's weapons bay.
So the fact that the H-6 platform is smaller or slower than a Tupolev 22M or 160 or a B-1 or B-52 doesn't really effect its value for the PLA.