It's not a huge, long-term business, for sure. There are only n doctors still running the DOS app in the entire market, and that number will only ever shrink.

It's not a huge, long-term business, for sure. There are only n doctors still running the DOS app in the entire market, and that number will only ever shrink. The lock-in is provided by the extreme niche-ness of the market the conservatism of these doctors. These floppy-guys have a unique position in the market, they know exactly what to do -- they are basically the only ones who can credibly pitch these doctors at all. Once they've bought a solution that actually works, is stable and compliant etc., these doctors are not going to shop around to save maybe $1-2000/year (or whatever you can charge in maintenance), it's simply not worth their while and the risk. Remember, these are not scrappy little outfits, they have cash to spend, and they will spend it if you can supply stability and continuity.