Interesting question...
I like HTC products in general, and I'm guessing that they'll do good things as the market continues to grow - but I think they need to make a decision to compete more aggressively where performance counts. The HTC Touch is interesting as an indication that other companies can potentially tread where the iPhone and iPod Touch are going. But at the same time, I don't think the HTC Touch UI is as slick as Apple's UI -- it's flashy, but doesn't seem to have the same level of elegance or user-centric design. I'm also concerned that most of the HTC products I've tried in recent years feel underpowered -- almost like trying to run Windows Vista on a 600mhz PC with 500MB of RAM. For me, the HTC products have been "almost there" -- enticing, but in the end, delivering a bit less than I had hoped.
From what I've read, the BB functionality would be an add-on for Windows Mobile. I think this will get us a bit closer - but I agree with your assessment that the biggest shortcoming is the OS. I keep wanting to think that MS will eventually "get it" and turn out a rock-solid seamless product experience, but if it hasn't happened in the last 27 years, I shouldn't be holding out hope that it will happen anytime soon. MS, for some reason, couldn't make push (or even pseudo-push) email work as reliably as BB over the last couple of years, and I suspect a big part of this is the way virtually everything in Windows (PC and mobile) is interdependent -- just an incredible number of ways for less-than-perfect software to mess up other less-than-perfect software, eventually ending with the need to reboot your PC or phone at least once a day.
The BB add on for Windows Mobile will, I'm guessing, give WM users a better email experience - at least in terms of reliable and timely delivery. But it's the other parts of Windows Mobile that concern me. As far as I know, MS hasn't yet completely solved their memory leak issues, or the creeping growth of the registry (PC and mobile). On a mobile device with limited memory, memory leaks and creeping file growth (like the registry) can bring the phone to its knees.
As much as I like the flexibility of the WM user interface (at least with 3rd party add-ons), it seems that Rim has had a better overall understanding of what it takes to keep the UI clean, simple, intuitively obvious and functional. Stability, functionality, reliability and immediately intuitive operation are the areas where BB has blown away Windows Mobile (in my opinion). I think that Windows will likely continue to improve, and the BB add on will help in terms of email delivery, but if RIM is smart, they'll also continue to improve and remain competitive.
So much will depend on the decisions that MS makes. Vista is a great case in point. It's pretty, and certainly has some "slick" UI features - even some nice system integration features. But it also is riddled with bugs, it's horrendously inefficient with computer resources (especially in combination with Office 2007), and the interdependencies cause it to hang far more than XP has in years. If MS makes similar decisions with the evolution of Windows Mobile, then it may remain too unstable to make a serious dent in RIM's business user base.
