Believe me, I and many other developers would love to write a true Creative Suite alternative, but it just ain't practical. Photoshop alone has been shaped by some very talented developers for nearly 20 years now. No one schmoe working by himself — or even a few schmoes working together — is going to replace that. The GIMP folks have come the closest (after 10 years of work), but even that isn't really a Photoshop replacement in anybody's mind.

Even if you did write an app that was as good as Photoshop for, say, a subset of Photoshop's tasks, I wonder if there would be much of a market. Most pros would still use Photoshop, I imagine.