tl;dr: Upload your contact lists to your service provider so that your service provider can block calls from people not in your contact list. If the blocking doesn't work completely from within my own device then it's going to be a no from me.
How would this account for (legitimate) blocked numbers or those that originate behind a switch? I get calls from recruiters from time to time that do this as a matter of course. Although I have no expectation the GOP would actually do this, I would add an asterisk to the policy... * If you use these lists for marketing to existing customers, we reserve the right to sue you out of existence. Kindly GFY.
If this gets widespread adoption, then recruiters and legitimate robocalls will come up with solutions. The problem I have is that if I block all unknown numbers, I'll also be blocking my bank/credit card/pharmacy or other legitimate robocalls. So I can't do this myself. But if everyone blocks unknown calls, these entities will be forced to come up with a solution (e.g. sending out emails instead).
The particular example you give of recruiters with blocked numbers rubs me the wrong way. Like you can call me out of the blue and interrupt my work day, and you're not even exposing your phone number? Good riddance.