Tom, that model sounds like it would appeal to both groups, as in not upset the casual user, and also allow people earning a living off of GP to continue to fund and improve the source (you). The way I see it is my income is based on a product you or any theme developer provide and support, there is a lot more value to it than you might think. I truly believe your sales will grow by adding a secondary premium level GP plan with an annual paid membership. I have used them all, had memberships with elegant themes, array, themify.me, and GP and the premium add-ons have them all beat.
I agree with everything Ted said, and I too have no concern over white labeling because I provide maintenance packages and maintain all the sites I build and host, kind of an all-in-one package. I have yet to have a client who wanted to manage their own site. If I ever did, I am very transparent about the themes and plugins I use, so I would just teach them how to use GP and beaver builder.