Even apps like Pixelmator don't quite get enough in there to adequately replace Adobe's offerings.
In fact, I can only think of Dreamweaver as the prime example of something that has plenty of superior competition, attacking from a variety of angles.
indeed, thats a very good example, but still, what we are forgetting is the insane amount of money it costs to buy adobe software, even if they have lesser market share, they sure make up for it. Supposedly, their customers are all the artist, for whom 1000-2000$ is like buying new clothes. This allows adobe to further seal its position, through spending on R&D to pull further ahead of the competition.
so we can say, adobe would not stand challenged until someone can beat them on this part, with a proper strategy.
Which is true for its current competitors, all of which only try to make the best from their current possibilities.
take gimp for example, it is nowhere near the dominance of adobe. its like windows-macintosh of the computer graphics world.