The fact is that the Librem 5 will be usable both with and without the facility to make calls via a cellular network. WiFi calling is a useful supplement to cellular calling. It’s a bit like having a femtocell in the building.
It would make no sense for me to use internet-based VoIP services for PSTN calls. I will already be paying for the ability to make and take calls via the cellular network, because that comes as standard with the most cost effective cellular data services. That includes the ability to use WiFi calling on supported handsets. Not only is the cellular voice call service effectively free of charge with my data connection (I can’t save any money by avoiding it), but it is also more robust to poor cellular signal conditions than VoIP over cellular data.
Given that I will already be subscribed to the ability to use WiFi calling whether i use it or not, being able to make use of it via a compatible handset will always be an advantage.