As a former 4e supporter I can say that I won't be coming back and quite a few of the people in my area will be in that boat. We gave the system a try and it wasn't to our liking. Many have sold all their books off and while I still have the first two volumes of the core three, as well as setting books for FR and Eberron, I find it very hard to return despite being mildly interested in the Essentials line.
Given what the new line offers I think it’s a little late in my opinion but had it been the initial version of 4e the game might not have lost as many players as it has. Might is the key and it is pure speculation as Markn points out cause we will never know. Will some of those who’ve left return, probably, but the real question isn’t whether Essentials will bring players back as much as will it keep them once they’re back? Sales can’t continue on an even course if you start losing the majority of your market and WotC won’t be able to get away with creating a new edition or product line every two years consistently.
Players remaining, along with the continuation of sales, are what will determine if the Essentials line is a success. I played 4e for over a year before I got tired of it and gave up. I’m a 28 year veteran of the game and outside of a two year lapse have never played anything else as dedicatedly as D&D until now. Pathfinder is my new game of choice, but until I switched that dedication fueled an average monthly expense of $50 on D&D that WotC no longer gets. As Dannyalcatraz points out the Essentials line won’t draw the same figures as a new edition would, but it might spike the market enough to make them happy for now. What WotC needs for it to be a success is for that spike to hang on longer than just during the initial release of the product line. Then and only then will we know if Essentials did well.