Of course this is pure speculation but given their pattern, If HTC sells as many Diamonds as they are expecting, I can see them putting some effort into a Windows Mobile 7 upgrade rom as they did with upgrading the original touch over other more recent devices. With that really depends on what kind of upgrade path Microsoft makes for the devices and if it's even doable, if Windows Mobile 7 lives upto what everyone is saying about it, may need to upgrade to a device specifically designed for it.
For me the Touch Diamond is the last step to reaching the perfect device. Has everything I've wanted, small form factor, good amount of program memory, all the necessary features, better screen with VGA. Only thing that could make it better is an even better (capacitive) screen with a higher resolution and of course, a much better more updated front end operating system, ala Windows Mobile 7.