I prefer a thicker phone (S9 at the moment) but I have large hands. I do hate holding this thing with my fingertips when I grab it from the sides.
No, I also like a device which has some bulk to it. The same way how I actively desire bezels on a phone display - better structural support, you can hold the damn thing securely without activating the touchscreen and it has more room for battery (which as far as I’m concerned is the most important thing in a mobile device).
Also, when someone says “mobile phone”, I immediately think “Nokia 3210”, which was 22 mm thick. My current device, a Samsung Galaxy S5, is about 14mm thick when you include its rubbery case. It feels OK, but would probably be better if it were a couple of millimetres thicker still.
BlackBerry Z10 and Passport had metal chassis. Felt really solid, and I don’t recall any heat issues with it unless it was charging. When it was charging you could feel it getting warm in the back(some composite backing). I imagine Librem 5 will be similar?