The market is very concentrated. If gmail, yahoo, microsoft and comcast announce that they will gradually increase the spam level on non-"new smtp" traffic, you can bet that adoption rate will stellar. Yes smtp will have to be around for a while but if you treat any legacy mail traffic as suspicious once it becomes <10%, I think it will go away within 3-5 years.
Most of those providers already enforce DMARC between themselves, which provides all the anti-spam benefit you could hope for from a new protocol. (In terms of protocol beauty, it's awful, but in terms of functionality it does what you want.)
If Gmail, Yahoo, Microsoft, and Comcast announced that they are making it more likely that outside email (from online stores, from individual Exchange installations, etc.) will be marked as spam when it's not actually spam, people will find new email hosts. Somehow.
If they just want to mark traffic as spam when it is, they're already doing that.
I am not so sure. Senders are more concerned about their emails not appearing as spam in gmail than the other way round. If all it takes is to update their software, I believe they will do.
