Obama and Hillary have debated 21 times. That's more than enough. It's getting to the point where they were debating about debating, not talking about the issues. He's right to want to to focus on bringing his message to the voters, rather than engaging in another pointless diatribe with Clinton.
If after 21 debates the public hasn't made up their mind about who they think the nominee should be, they're never going to make up their minds.