I agree with you, with the caveat that you include "given its current trajectory." Is, say, Sparrow better? Maybe. Will someone release an awesome new email client in the future that's better than Sparrow and Thunderbird combined? Definitely. But with infinite time, Thunderbird will never become Sparrow (or Gmail or the next big email client).

Any further features for Thunderbird can be developed as an extension or as a fork (like, say, Postbox -- which is actually really more of an extension rather than a fork, as far as I know).