[Brightcolours]

Let the team review some of the lenses, and than we will discuss again whether or not is it worth it.

F1.4 with the same optical performance will be huge, and if you are shooting at 5.6 anyway, it makes no sense... F1.8 is a great compromise on FF.

If you "are shooting at f5.6 anyway", you should question the need for FF in the first place.  MFT or APS-C will offer what you need even without any f1.8 primes.
If, however, you would like the possibility to shoot for instance 85mm f1.4 once in a while, you can only achieve that... with a f1.4 lens. Sure, 85mm f1.8 will be enough for many, but not for all. Same goes for 50/55mm f1.8 vs f1.4/f1.2.

In short: f1.8 "if you are shooting at 5.6 anyway, it makes no sense..." either.

Been there, done that, there is a difference in IQ, period.

The difference beween theory and practice, on theory, is none.

There is no real difference in IQ, actually. If I take 2 equivalent images, it is very difficult to say which is which. At the same time, I can't really tell the difference when I take an image at ISO 100, 200 or 400 with my 6D.

In theory and practice, there is no significant difference in IQ. Unless one is allergic to shadows or into pixel peeping instead of making photos...

You are right - if you are posting on Facebook, the compression and size reduction will take their toll. If you print or do something meaningful with your camera, the difference can be obvious is some scenarios, and not so much in others.

One additional thing to consider - some systems just don't have the same level of optics, because they are focussing on compactness, not on ultimate IQ.

Performance takes its toll. Sigma Art 40/1.4 is bigger than Voigtlaender 40/1.2 Nocton, yet, it is slower. But it's hard to compare both, especially wide open.

Once again - cannot defy physics. Every lens is a matter of design decisions what tradeoffs to make. Nikon decided to put IQ first. You can still buy Canon EF 50/1.8 - it's light, compact, and cheap. Mine fell apart in less than a year, and it was sitting in my bag, at home most of the time.

I am right if you print the images too. There are no special properties to sensels if they happen to be in a FF sensor. And I can't see real differences in real images between for instance ISO 100 and ISO 200. Or 400 for that matter. So even when equivalent ISO settings are not needed, the difference between ISO 200 on APS-C and ISO 200 on FF will be negligible in print.
All depends on the lens. Yes, the Nikkor Z 50mm f1.8 is a nice lens. So is the Canon EF-M 32mm f1.4 STM.

It is fine to argue that the Sigma Art 40mm f1.4 will make a difference to a Canon EF-M 24mm f2.8 STM... But then you are comparing (extreme) lenses, not the sensor format.

So yes, the real advantage of FF is that you can easily get less DOF, and that is its main advantage. And if there is a special lens that happens to be FF, that you feel you do need, again that may be a real valid reason to choose FF.