It'd probably become like SPF/DKIM - somewhat spammy companies who still have a genuine mailing list and will still opt you out if you ask (or even click 'Spam'

It'd probably become like SPF/DKIM - somewhat spammy companies who still have a genuine mailing list and will still opt you out if you ask (or even click 'Spam' in gmail etc via FBLs) will go to the effort to implement it, but random viagra spam being pumped out by botnets likely won't.