It appears that Google groups is not as ready as Groups.io to _preserve_ a group. Mention is made on the linked Stack Overflow page of a rights issue keeping the Personal Groupware download from being uploaded to Google Groups.
So... One might sort of _archive_ with Personal Groupware. (And any of us might, it seems, fetch our own copy)
But to preserve the group's function, it's Groups.io with a $110 cost per account (So if you've several groups, it might be seen as less per group).
I do expect I'll be taking the steps above, both the PG archive and the Groups.io migration.
But I'd love to hear from the list owner. I'm just a deputy mod. It's not about the money, it's that this all ought to be his call.