I understand the sentiment, but my guess (similar to what @mwra said) is that this would put unsustainable strain on the Eastgate development resources, to come up with something for which there are already superb alternatives – and would divert time and resources from continuing to refine the aspects of Tinderbox that are genuinely distinctive.
(Eg, Tinderbox could never be as powerful for large-scale writing as Scrivener, nor keep up feature-for-feature with mainstream normal word processors like Pages or Word; and for normal text edit, you can just copy-and-paste from BBEdit or TextWrangler or whatever.)
So in a world of constrained resources, I would vote for Tinderbox concentrating on what is unique about it.
PS But in sync with your original suggestion, and slightly in contradiction to my larger argument, I am glad that TB’s editor supports some basic and useful rich-text functions like different fonts; bold and itals; list-making; and so on. My view is: these are good to have, and good enough for such composition as I actually do within TB. When I want more advanced features, I personally switch to a “real” word-processing program. FWIW.