Define commercial success. Most of the early American Idol winners have had record deals and released multiple albums, which would imply commercial success over a "one-and-done" deal. Of the 20 Idol winners, 5 have been nominated for Grammys, which isn't 100%, but 25% Grammy nominations is no small thing.

The weird thing about the music industry is that you can be commercially viable via your tour schedule, so album sales/production isn't really a good marker of success.

But I'd try comparing your question to Billboard top 100 artists. That's a real question - how many people retain success in the industry after "making it"? I bet you'll find not many continue on the popular success, but find a way of making a living doing what they love.