I really doubt the graphics would have much of an impact; after all (IIRC) they haven't really been updated in forever, meaning most of those 12 years were spent on the programming.
Graphics would have an impact on development - not having graphics makes it trivial to add large quantities new content, things, creatures, etc.
Even simple sprite graphics would require to be created/modified/maintained at every modification, thus increasing the barrier/cost of such changes and causing less changes/experiments to be done.
Currently, the author publishes the changes, and fan spritepack maintainers do all the work of maintaining those sprites and their relation to the raw datafiles - it's a lot of continuous work; if Toady would have to do all of it himself then it wouldn't scale.
