Many business models these days are 'x' on the web, where 'x' really does not improve by being on the web and practice management is a prime example of that. If you don't believe that try to explain to a dentist why they need redundant internet access to keep their practice running in case they have internet problems when right now they can do so just fine without any access at all.
Web apps have lots of upside but they also come with a set of requirements that is not always practical or even desired.
On top of that you discount domain knowledge which I think is a very important part of any successful business in an area like the one you describe.
If customers expect to be able to book online (and as a user of services like ZocDoc, FreshDirect, Instacart, and Homejoy I do expect these things now), then providers who do (theoretically) provide these things will have an advantage of those who don't.
Not going to argue for or against the 'upside' part, as I do think that is debatable, at least in the short-term. A big part of this is avoiding 'downside' by not adapting, IMO.
You do have a market: new dentists. But old ones are just happily going to roll along with their gear until either they or the gear dies of old age. And then they or their heirs will sell the practice to a new dentist, who will have to wonder about re-keying all the old data into your shiny new software or to focus on what he's best at: dentistry.