It seems that there are few/no software that produce descriptors using H, while few accept it during parsing. As neither of the reference implementations of Bitcoin Core nor rust-miniscript support H at all, it seems to be better to remove H as a hardened indicator from the BIP rather than updating all of the existing software.
I asked about this on the mailing list last week and have received no responses. There was also some discussion in https://github.com/bitcoin/bitcoin/pull/32788.