I’d be all in for hardware keyboard case, feels like a good compromise. I doubt Librem 5 will be a very thin phone, so nicely built slide-out keyboard will add at max 7-9mm.
Nokia N900 has a decent keyboard, it is there on demand and also can contain additional stuff, like battery.
Besides that, in case of dealing with any sort of a Linux device (esp. those where you can install various distros) the chances are you might need a keyboard during booting stage.