The 80/20 rule of FHIR does not mean that 80% of people must approve the base resource specification. It means that the base resource specification will accommodate 80% of use cases. You present FHIR's flexibility as a weakness, it isn't. It is a tradeoff. Although you lose the comprehensive coverage of OpenEHR, you gain flexibility to respond to new scenarios.
The problem here is that when you define a "minimal" data set and ask people to "extend" it, chaos ensues. As we are seeing with the proliferation of different FHIR profiles. Each country has it's own take on how to model "blood pressure" or "Medication Request". openEHR on the other hand has a maximal dataset and asks the users to constrain and use a minimal version of it. This to me makes all the difference.
If I were building a system from grounds up, I will start with FHIR and then try to establish the wholistic model later and use both together. If you are worried about change requests, well welcome to IT. This is true even is you use a wholistic model
