I doubt that GIMP will ever overtake Photoshop. Adobe has the money to employ (and does employ) hundreds of experts in their fields to work on Photoshop for 8 hours a day, 5 days a week, 52 weeks a year. Although GIMP is very impressive as an open-source project and a massive testament to how far the free software model can go, it is still, at the end of the day, made by a ragtag band of (mostly) amateurs volunteering their time. Adobe, by brute force, can deliver a higher-quality product just by having the resources to employ the best people to work for them.
I love GIMP. I use it for all my image editing needs and would never consider giving a dime to Adobe. But I don't do it for a living and I respect the opinions of those who do when they say that GIMP isn't a good replacement for Photoshop.