Hey Sargon,
of *course* Cocoa is the way to go. It was clear from the beginning that Apple wants developers to port existing apps to Carbon and code new ones on Cocoa. Fact might be that devs won't and Apple is improving the Carbon API (not to the extent that it will be easier one day coding for Carbon than for Cocoa but performance-wise).
Why you can't use MP3s & ICQ at the same time I don't know. I don't see such a big performance gap between Cocoa/Carbon as of now (10.1.1, 9.2.2). iTunes hasn't been a 'performer' really, of course, so far.
But people don't want a Carbon version of Outlook Express, because they like Carbon more than Cocoa, they just see that MS won't possibly write a whole new E-Mail app in Cocoa right now and give it away for free. Because Apple already did. And I must say: Apple's Mail.app is great for daily E-Mail business. Also for thousands of E-Mail (to an earlier post).
But if you want more features than Mail.app delivers, the competitors are Carbon based right now. And Entourage makes a good competition and can even import mbox'es.
Maybe OmniGroup should do a PIM (Mail, Contacts, Dates, Palm integration, whatever other feature comes to mind). They make great software and *do* use Cocoa.