I agree with the points you make. I have tried for many years to get people and companies I know to move from proprietary protocols and products to free and open source software.
The question I am frequently asked is, can they migrate their data (and also, if the they are unhappy later, can they export their data from the open source product they have chosen).
I also realize that NS and SoGo is not a direct Exchange replacement in the sense of Outlook connectivity, but it does offer the same functionality and could replace Exchange if users used a web client such as SoGo or WebTop (or Thunderbird and Lightening). In fairness to Outlook it does offer many ways of exporting data as well as pst files. However if we want open source groupware to get a greater "market share", data import /export, in my view, needs to be simple and reliable.
I suspect that many installations of open source servers and group-ware are in places that are trying to escape vendor lock-in and high licensing costs. Many such places have lots of historical data that need to be migrated.
Just my thoughts,
Tom