Taking the case of a company which works on SharePoint since a while and have engineers trained to develop full-trust solutions. So, they develop full-trust solutions to add functionalities to its farm. Often, functionalities developed are not end-user, not read-only. The company also store sensitive data and they don't want to put their data on the cloud.
In this case, is there a real interest to trained engineers to add-in and SPFx in order to develop new solutions to replace their full-trust solutions ?