Changing the drummer would be "easy" - they did it 8 years ago, it went well, they'd bring in someone else without even the issue of "damn, the drummer was a founding member".
This means no disrespect to Mangini of course, it's just a "cold" analysis of the situation. In DT the most replaceable member is the one who's last arrived.
James not doing it anymore for whatever reason would be a way more difficult blow to the band.