Well I think it is a more complex answer than simply arguing the show was cancelled. Afterall there are good or promising series that are canned while subpar stuff goes on for years and years.
I'd also point out that while ENT had a rough first two seasons the show had a marked improvement in seasons three and four. Sure Coto pretty much guided season four but season three was a lot of their vision and played strongly into Brannon's strengths-high concept sci-fi elements, big spectacle imagery, action and plot-driven storytelling.
I think a large part of ENT's problems were the writing and the characters not resonating with the audience. And even the oft-cited savior Coto couldn't really make the characters come alive so he instead focused on the stories and continued the plot-driven approach.
The third and fourth seasons were much better than the first two, but in terms of quality Star Trek, they were far from flawless themselves.
Despite the upswing they were unable to attract back any of the viewers that they'd lost over the course of the first two years, and eventually lost even more.
Cancellation's a harsh yardstick, but ultimately is a fair reflection of fans voting with their feet (or fingers on the remote in this instance).
Shows like Andromeda, Mutant X et al survived for as long as they did because they played to their respect audiences, and although they were pitiful series' in themselves, they did that very well. By the time Voyager, and later Enterprise came along, they already had an inbuilt audience, and could do nothing to maintain its interest. Stories over the first two seasons were relatively simplistic, often recycled, and as you've said, did very little for character development in the long term.
Enterprise was just far too bland for its own good. By the time it realised that, it was far too late.