I don't really relish a future where one browser engine dominates the way IE has in the past. I fully expect and hope for more viable mobile engines.
Even still, jQtouch (at least, I haven't messed with the Sencha thing) is very iDevice slanted. There's "webkit" and then there's "webkit on iOS", and most tools are targeted specifically at "webkit on iOS".
The thing the differentiates WebKit from IE in a good way is that it's completely open source and it's used and built by several competing companies working together toward a common goal.
It's also behind both IE and Firefox in various ways. You're right that it would be a better dominant engine than IE was, but it's still not as good as having a nice three way competition for browser share.
Even still, jQtouch (at least, I haven't messed with the Sencha thing) is very iDevice slanted. There's "webkit" and then there's "webkit on iOS", and most tools are targeted specifically at "webkit on iOS".