I suppose it is a niche, for competitive gamers that either need mobility, or lack the space for a desktop. Personally I enjoy gaming, but a gaming laptop isn't for me, regardless of the price, because I need the laptop for work as well, and, with few exceptions (such as the Lenovo Legion), pure gaming laptops have a keyboard layout that isn't suitable for many work-related tasks (majority have small cursor keys, and/or no numpad and/or no dedicated keys for PAGE-UP/PAGE-DOWN/HOME/END). My kids like gaming too, but also need the laptop for studying, and, again, the keyboard layout isn't suitable, even if I could afford to buy them pure gaming laptops.
As for the screen size, while I agree with George that many laptops larger than 15" aren't really that mobile (with the exception of the Zephyrus, the remaining laptops mentioned in this article are all between 3.5 kg and 4 kg together with the power supply), gaming laptops seem to be following the trend of moving to 16:10 aspect ratio. I guess I can understand why (as a laptop manufacturer, you'd rather use the same screen aspect ratio for all models), but while the 16:10 aspect ratio is preferable for work, for many types of games (such as first-person and shooting games) it increases the overall screen size (making the laptop larger) without much benefit, because for these types of games you prefer wider field of view horizontally. So what was for example 15" in 16:9 would have to be 16" in 16:10, to compensate.
Hence the market for these devices, IMHO, is more portability than mobility. But I wouldn't say it is desktop replacement, because for me a defining characteristic of a desktop replacement is having as large screen as possible (as you are replacing all components of a desktop, including the screen, in a portable package). So in 16:10 I guess that would need to be 18".