No, I actually think Justin Sun won't oppose it. He's said he is not going to vote witnesses with his stake. He should, if he understands how witness votes work, support this strengthening of decentralization of governance on Steem. He has said he supports decentralization.
I think it's definitely long past due, but I don't think it's too late.
He did not make a firm promise to never use his votes, it was a soft, clearly temporary statement.
Yes. However, this change does not prevent him having enormous influence over governance, it only prevents him from having the ability to install 30 witnesses at will. I don't believe he will oppose the measure when compared with the threat some are making to fork the blockchain.