I'm not an expert, but unless there are G+ specific exceptions I've missed, I believe that the current wording of their terms-of-service would prohibit them from engaging in the efforts to archive G+ pages.

This it was it says about the rights you give Google by/if you are using their services:

"The rights you grant [to Google] in this license are for the limited purpose of operating, promoting, and improving our Services, and to develop new ones."

The only rubric archival could possibly fall under would be "promotion", but that feels marginal at best.

I see your point, but it’s hard to believe the Internet Archive has more ability to do this than Google does, since they were granted no rights whatsoever.

Seems like whatever legal theory the Internet Archive is using must apply to everyone, including Google, unless Google waived whatever that is.

Or just freeze it as-is, read only...