I've looked at Contributor and found it weirdly limited. Google doesn't need publishers to sign up for Contributor, because they are already all signed up for AdSense! The most logical way to implement Contributor would be essentially as a personal buyout of AdSense ads: if you are a Contributor, the AdSense JS checks that and then simply doesn't run, and assigns a fraction of a cent to the publisher in lieu of a displayed ad. Everyone gets what they want and this immediately gets you coverage of millions of websites instead of, like, 10.
That's how the original Google Contributor actually worked, it would remove Adsense ads from all websites running AdSense. You could even choose what to replace them with (eg pictures of kittens). Matt Cutts posted about it here in 2015:
Apparently that model was a huge flop and people just didn't want it. The new version requires publishers to remove all ads (not just AdSense), which helps since many newspapers seem to have dropped AdSense & switched to Taboola & other ad networks. Which might be one reason Google is trying it again.
(I expect the new Contributor will fail too, though. I think the majority of people just don't want to pay for online content.)
That's a pity. I expect the new optin model to work much worse; not sure why they're bothering... Patreon, IMO, shows people are willing to pay for online content but the exact method and experience matters a lot.
