I’m not arguing there is more important stuff, but DUs would enhance the language on a wide, fundamental level, accelerating a lot of other advancements.
The only thing I really missed, has been adopted during the latest years, pattern matching.
Languages are not used in isolation, great IDE experience, and having mature libraries for every use I can think of, is more valuable than grammar and semantics.
Also a reason why I would rather do FP in C++23 than Haskell, even with all the warts and paper cuts it entails, ecosystem.
If only the tooling was at the same level as C# and Java.
Additionally, they add friction to a development stack, now everyone needs to be confortable with two language stacks, and most of the time it isn't really worth it.
C# has done great in the industry for the past 23 years without them.