[Brightcolours]

To reply to your statement "As BC states, what good is it to go FF if not having the advantage of shallow DoF and great low light performance", read my reply to BC.

There were many film cameras that were much smaller than DSLRs.

Regarding slow primes (between f2.8 to f4), you're missing the point.
I'm sure many people would be interested. Nobody says that's the only type of lenses one should use. It's all about choices. One could mount fast large glass or slow, compact glass. The point is that you use the same system for both scenarios.
When I shoot landscape, I don't want an f1.4 or even f2.8 lens since I'll stop down the aperture to at least f8 (FF). Consequently, a small 20mm f4 or even f5.6 prime could be very useful.

The smaller cameras were all from the "yikes, did we really enjoy using those" ilk, with sharp edges, heavy metal construction, no ergonomics to speak of (and none had sensor electronics boards, LCD, big battery, so they could be slim and smaller). Yes, my Nikon  Nikkormat FTn was smaller than my 6D. And my 6D is way nicer to hold and use. But of course, it has to be fatter because of the sensor and LCD stuff. And luckily it has a nice grip.

Better just look at an APS-C body, with a wide angle converter for shallow DOF options?

And btw, my small Voigtlander 20mm f3.5 is very useful, and is very small. So I understand that idea. But of course, a Samyang 20mm f2 offers the same on APS-C, and gives better close up results.

Another thought... A 16-35mm f4 lens may be bigger than a slow prime... But it gives a range of focal lengths. Easier to use and carry than a 20mm + 24mm + 35mm prime. Then going back to APS-C.... For small aperture shooting, my EF-S 10-18mm f4.5-5.6 IS STM is way smaller and super lightweight. So again, better look at APS-C with wide angle converter option?