First the solution doesn't really fulfill all of imaps use cases (especially push mail) but that is unimportant.
What is important is that the IMAP protocol isn't difficult, and I don't think that is what limits "email innovation." It is more so that it has been around since the 70s and email clients haven't been able to remove themselves from their past. It isn't the protocol limiting them from moving forward, it is the shear number of other clients that wouldn't support any of the "innovations" one would try to bring to email.
I'm very hopeful for Letters, a project just taking off under the leadership of some pretty big names: Gruber, Rands, Brent Simmons. They're hoping to collectively create a "lean and programmable IMAP email client, with plugin and automation APIs, designed for developers and power users." While still sparse and very early-stage, their vision document is ambitious: http://pastie.org/785269 If something comes from this, it'll definitely be a step forward to modernize email clients.
What is important is that the IMAP protocol isn't difficult, and I don't think that is what limits "email innovation." It is more so that it has been around since the 70s and email clients haven't been able to remove themselves from their past. It isn't the protocol limiting them from moving forward, it is the shear number of other clients that wouldn't support any of the "innovations" one would try to bring to email.