I think it makes sense to reuse a VLS-based missile design for bombers.
1- A larger missile is unnecessary if range and payload are deemed enough
2- A missile that is launched from an aircraft would have kinematic advantages, making it easier to achieve the needed range and/or payload.
3- If a bigger rocket motor is needed and rocket thrust and structural rigidity are enough (these are rarely a problem in most rockets) it is relatively easy to elongate a missile compared to designing a new missile.
4- Reusing the same missile saves a lot of money and time
5- A larger diameter missile may shorten the range of the carrying H-6 because of the extra aerodynamic drag caused by its larger diameter.
I believe they calculated that they could achieve the intended range and payload with a variant of the missile for VLS.
I have nothing against the idea of reusing a VLS based missile for air launch.
But I hope you can recognize that there are ready made counter arguments to all of your points, for example:
1 - what if the range and payload of a reused VLS missile/RV is insufficient?
2 - what if the kinematic advantages of a air launched VLS missile/RV is still insufficient?
3 - what if you want a bigger RV itself, rather than just a bigger first stage?
4 - what if the money and time is not enough to be worthy of the capability that you want?
5 - what if a reused VLS missile/RV has insufficient range to achieve requirements to offset whatever reduction in drag the missile may have compared to a larger weapon?
Now, I'm not inherently against the idea of adapting a VLS launched weapon for air launch. In fact, I'm quite a fan of the concept.
However, what I'm arguing is that I do not think the
VLS launched "YJ-21" shares common parts or RV with the
ventral H-6N launched AShBM/HGV, because I do not believe the dimensions and proportions of their respective RVs are identical. In fact, based on the external proportions of the H-6N launched AshBM/HGV, I believe its RV is significantly larger than that of the RV of the "YJ-21" if we keep in mind that the RV of "YJ-21" has an overall diameter that must be well below 0.85m.
If they really wanted to adopt the VLS launched "YJ-21" for air launch, I believe it would have made more sense to adopt the weapon as they were for launch under the wing hardpoints of H-6K/J/N, all six hardpoints of which are rated for KD-20 and the central four are rated for YJ-12s.
That would be a much more sensible use for the size of the weapon, and allow H-6K/J/N to carry between four to six of the air launched "YJ-21s" under its wings.
Large fin sizes don't appear to be a feature of hypersonic missiles, so I don't see a significant tradeoff to use the same glide-body warhead for a VLS and air-launched version.
So if you already have a hypersonic missile developed to fit in a VLS, you might as well reuse what you can for an airborne version.
Of course, that doesn't stop you developing a larger missile which would not fit in a VLS, but it makes sense for this to be in a higher weight/range class.
???I never said anything about fin size -- I was talking about the size of the re-entry vehicle/terminal warhead stage, and emphasizing why we shouldn't believe that the "YJ-21" RV is the same one as on the H-6N launched AShBM/HGV simply because they look superficially similar, because there are a large number of missiles and RVs with that kind of configuration.
In terms of size of the RV, think about the capability difference.
There's a difference between a re-entry vehicle whose diameter must be sufficiently small to fit inside the 0.85m diameter of the UVLS -- versus a re-entry vehicle that is literally towed ventrally under the main fuselage of a H-6N that has been specially redesigned and produced to carry a big missile.
Think about just the difference in warhead size alone.
I have nothing against commonality when it is appropriate -- and if they were to ever adopt the "YJ-21" as a weapon that H-6K/J/N can carry under its wing pylons I think that would be immensely sensible. The "YJ-21" is a weapon well sized for underwing hardpoints.
But it would be an immense waste of space to use the H-6N's ventral fuselage hardpoint for such a small RV as on YJ-21. A larger, clean sheet design or even a RV based off that of DF-26, would be much more valuable.