If you want developers to migrate from VSTO to OfficeJS, you have to provide equivalent functionality. One of the biggest blockers for us is the complete lack of support for drag and drop of any kind. We have a VSTO Outlook addin where users can drag e-mails to a task pane. Users can also drag e-mails out to for instance save them as a file on the desktop. This is completely impossible to do in OfficeJS today and the alternatives are major UX downgrades, so a no-go as well.
If you want to change the perception that OfficeJS is only for simple toy addins, you have to match the capabilities of VSTO. Until that happens, OfficeJS addins will remain restricted to simple addins and not the complex and feature-rich addins that have put Office where it is today.
