The cost of making such a change is the switching cost. But there is another danger with this approach. One's fortunes are inextricably tied to the progress of the manufacturer. And in medicine, where decision support systems may assist in all kinds of outcomes for patients the idea of proprietary knowledge that is locked into a system away from the scrutiny of peers and colleagues is untenable. Classification systems that cannot be adapted by clinicians for their own practice are likely to prove intolerable and unworkable in practice.
Proprietary systems do provide methods for extending their classifications with custom codes.
For example, the popular EMIS system for general practice in the UK allows general practitioners to extend the Read codes to include their own personal codes. What such a system does not provide is any ready method of polling for these changes and revising the classification process in the light of experience in use of all users. In the age of the internet this seems perverse. Such user involvement in the revision and update of information systems in daily use requires a covenant on the part of the owners of the system that their efforts will not be wasted. Such problems are essentially political not technical, so it is unsurprising that the solution is also political.