I would probably approach it this way: Sit down and have a frank discussion with the boss.

"I've been thinking of how we can find ways to be more productive and take on more business. Are you happy with my work? Perhaps if I began supervising a team of young, new, eager developers, you would have more time to go after additional business, and my team would have more time to take on the new projects you are able to bring in. Our current clients would be happy because as issues arise in the products we've delivered, I'll still be available to bring a swift resolution. And this company would be happy because we can take on more clients. ...it's a better separation of concerns, and probably a more profitable use of my years of experience."

But then I am not afraid of the outcome of that sort of conversation. Where I'm from, employers appreciate a little ambition. Past experience has proven to me that the "let's build this business" face to face meeting tends to work out favorably.