I agree that a 50 f/1.8 provides pretty shallow DOF and in many situations f/1.4 may not make much difference. In fact, sometimes it can be counterproductive (unless you actually want only one eye in focus). However, as you note, in some situations, f/1.4 does make a useful difference. Personally, I think you can really see that with 85mm lenses. For example, for head and chest portraits, generally I would not shoot wider than f/2 as I don't think going wider makes much difference to the bokeh and I like the extra DOF for my subject. However, for a looser full body portrait, I think 85 f/1.4 can produce a result which is, to my eye, better than a f/1.8 produces. (I am talking in general terms, of course. The design of the specific lens can make a difference too.)
Anyway, coming back to 50mm lenses, I can understand people being happy with f/1.8 but equally I can see some people wanting the wider aperture to help in low light (so as to limit ISO required for a given shutter time) and/or to allow more blur / shallower DOF. I don't know enough about lens design to comment on what design options there might be, but I note the RF 50 f/1.2L is 950g and 10.8cm long, the Sony 50 f/1.2 GM is 768g and 10.8cm long, and the Sony 50 f/1.4 GM is 516g and 9.6cm long. Further, from what I have seen so far, the IQ of the 1.4 seems to be right up there with the f/1.2 lenses (subject to the fact it only goes to f/1.4 of course). So, as much as I like some of the photos I have seen coming out of those f/1.2 lenses, I have no interest in buying one. However, the 50 f/1.4 GM is much more interesting to me, because of its lighter weight. That reflects the circumstances in which I usually use my camera and my personal preferences, of course, so YMMV.
Looking at the 50mm f/1.8 lenses though (Canon RF or Sony), I am not too interested in them. At one stage I owned by a Sigma 50mm Art and a Canon EF 50mm f/1.8 STM, and I knew someone with the Canon EF 50mm f/1.8 II. The little f/1.8 lenses just weren't in the same league for IQ. To be fair, with simple backgrounds and particularly stopped down, they were OK. A portrait of someone with trees / foliage in the background? The 50 Art was just so much better in that sitution, and even when the lenses were shot at the same aperture.
However, in Sony land, there is the Sony Zeiss 55mm f/1.8. That lens is 281g and about 7cm long, and it seems to have a reputation for being sharp although a mixed reputation for bokeh, and it is known for having quite a bit of longitudinal CA. I haven't used one, unfortunately, so I cannot speak from personal experience. From what I have seen online, the 50 f/1.4 GM seems maybe a little better (cleaner, slightly nicer bokeh) but personally I would be more likely to try the 55mm I think. In other words, I would be willing to trade the wider aperture and better IQ for smaller and lighter given I think I probably could be happy with the IQ of the 55mm (although I'm sure I wouldn't like the LoCA when it showed up). That also reflects where a 50mm lens would fit in my kit though (I already have a 35mm f/1.4), and the fact I like the idea of the focal length being slightly longer than 50mm. Obviously, again, YMMV. (In fact, I can see myself picking up a 55mm, or perhaps the Sigma 65mm f/2, at some point. Or I suppose I could switch my 35mm f/1.4 for a 35mm f/1.8 or 35mm f/2, in which case I would be very keen to get the Sony 50 f/1.4 GM. Given I like 35mm as a general purpose walk around prime, having a smaller, lighter, 35mm would not be silly. I just don't think I can bring myself to sell my 35mm f/1.4 GM though, especially since it weighs 524g which isn't too bad.)
Anyway, my primary point is that in many ways the 50mm f/1.4 GM does seem to occupy a middle ground between the 50mm f/1.2 and f/1.8 lenses but with IQ pretty close to the 50 f/1.2 lenses. I can see that middle ground being very attractive to some people (although what proportion of buyers, I can't say).
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