That sounds like a very logical approach. I'm personally very glad I bought my Ethervox years ago, because it was at a point in my life where I didn't have much other responsibility :) These days, it'd take an act of Congress to justify a purchase like that again. If I was gigging regularly, it would make more sense, I suppose.
I like the idea of a theremin fund. Contribute a little each time you make money with another theremin, and eventually retire the first one (and keep it around as a backup!).
I'm guessing the 'Pro will have a much longer production run than the Ethervox did, mainly because it's so popular and so much more accessible to musicians. That said, I can't guess how many more years Bob will be building theremins (or anything for that matter).
Also keep in mind that the 'Pro is still really new, so there isn't much of a market for used instruments today. That will probably change over the next year as some people realize their theremin has been sitting around unused or they want to trade it in for a different instrument.
Keep your eyes on eBay :)