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.