Tbh I'm surprised you would see the UVLS as space inefficient -- personally I see it as quite an elegant solution for being able to accommodate large diameter weapons while also being able to hot launch weapons if they choose so for individual cells, without needing the extra space needed for a dedicated common venting system.
It means each individual cell had the option to be cold or hot launch as one sees fit.
I suppose it's because we have a slightly different view of the relationship between H/AJK-16 & UVLS.
Personally, I used to see UVLS as a replacement for H/AJK-16, but now I think of it as a complement for H/AJK-16, specifically for launching large, cold-launched missiles. Meanwhile, H/AJK-16 will handle the smaller, hot-launched missiles.
In this case, HJ-18 is an exception within PLAN VLS missiles, being so big yet also hot-launched. So I'm thinking if PLAN had known they'll have a large, hot launched AShM like HJ-18, they'll design H/AJK-16 to handle it as compact packaging saves space. After all, they definitely knew about USN Mk. 41 and its modularity while designing H/AJK-16.
BTW, another pet peeve of mine when talking about PLAN VLS is the PLAN SAM systems that fill them. HHQ-16 is hot launched, while HHQ-9 is cold launched. They also have different guidances, which further complicate compatibility.
Compared with USN's SM-2MR/ER, SM-3, and SM-6, which are all hot-launched and compatible with Mk. 41, PLAN SAM selection is just messy. And let's not talk about how land-based HQ-16 actually went cold-launched...