From what I have seen, Phosh is a fine graphical environment for a phone. However, once convergence as a desktop PC starts becoming a reality on the Librem 5, I am going to want to replace GNOME Shell with Cinnamon or some other GTK-based desktop environment.
I really hate “innovation” in desktop environments. After trying many different desktops, I always decide that the first graphical desktop that I ever used, which was Windows 3.0, is the best design. IMHO, the only useful innovation since then has been the addition of a search bar in the menu.