The ones with outdated UIs have a captive market because the market is so small that no one bothers to compete. There are only 200,000 physical therapists in the US.
A market of thousands of customers who pay yearly for licenses and maintenance could be a great business for a small company to get into. It just doesn't have the growth potential to be a unicorn company that a VC would be interested in (which means you don't have to worry about competition from VC funded companies).
But even getting a quarter of those therapists to all pay $50 a year would be a pretty good businesses imo.
Is it realistic to expect to get 25% of a market you are newly-entering? I would say 1% or 2% would be more realistic, especially given the incumbents.