Google Groups DOES have reply-any but it is well hidden

Google Groups DOES have reply to any for USENET posts

The feature of replying to any post in a USENET thread does exist in Google Groups, but it is confusing. You get reply only to the lead article if you hit the big red reply button at the top of the page, but you can reply to the current reply. You must use the left-arrow icon at the right side of the page. This is confusing UI design. It may be there to discourage multiway replies while still sticking to the RFC design of USENET posts. This may be misleading.

So, possibly the reason Google Groups is panned by USENET users may be because of the low number of newsgroups offered, and there are free NNTP newsgroups out there.

On Facebook I said that if a domain owner is reluctant to accept the full load of text-only newsgroups from other NNTP sites that it could at least create its own private hierarchy.