Well, Jolla also used a whole lot of existing software left over from the MeeGo effort and from general Linux ecosystem. But it's true they also had to develop a lot of stuff themselves (UI layer & default apps).

Still well be interesting to see how will Librem handle that - while Gnome Shell should not be that hard to use/change for a mobile device, you can't just run Evolution or Libre Office on a touchscreen phone with the current UI.

So a lot of existing software would still have to be adapted to be usable on a mobile device (basically the same problem Jolla had to solve) and someone needs to do that or people would not have any apps to run on their Librem phones.