I want them deindexed and claim back the link juice and crawl allowance.
Deindexed - Yes That's the exact function of the noindex meta tag.
Link juice - No Link juice still accumulates for the URL as long as there is a link pointing to it. Even if you make the link nofollow, the other links do not reclaim any share of link juice whatsoever.
Crawl allowance - Just a bit In order to even see the noindex meta tag, googlebot must crawl the page. It may crawl less frequently after it verifies the noindex a few times, but it must continue to crawl.
I doubt there will be any downside to this action whatsoever, unless for some reason those pages were getting significant search impressions or traffic. And from what Google has been saying, there may well be a positive effect of all these profiles were somehow tagged as low quality and that bled over into the rest of the site.
If that did happen for such a standard thing as a profile page, then this algo has more trouble than I thought.