The fact that you consider your wife to be your best friend says it all. Obviously, you are doing something right, and I wish you both all the best.
Only way it has ever worked for me is when i knew i could never be with the guy in real life.,sadly he did catch feelings.
It has never worked for me, not to say that it can't. Usually one side catches feelings which obviously messes the orginial arragnment up. Why woulldn't you want to transition to a full-time relationship with someone you have great sex with but knew you were also good frineds wit, thats what a lot of people are looking for?