Handhelds need a long way of improvements to be actually good. If we skip Nintendo Switch (which isn't perfect, but somehow is better as a whole product), then every single handheld console released in the last years runs hot, battery lasts for maybe 2 hours, and is still too slow for many games (or you like to play at 720p). Additionally, there is also what mackerel mentioned, some games are really annoying on small screen, not to mention office applications.
I see that computers are getting smaller, but displays are getting bigger. At work, we only buy 27" right now, and our clients pick 27" for office work, too. So you can get a well-performing NUC or other small PC, but you still wish to use it with a large display, not a 7" handheld that still would need a mouse and keyboard to be useful for most tasks.
ASUS "plugless GPU" still requires an additional slot on the motherboard to deliver power and the motherboard needs to transfer that and have additional plugs. So it's not that it doesn't have the plug, but it has it somewhere else. They should simply fix the problem with high-power plugs, so only one would be required and so it was 100% safe to use. Later it doesn't matter if you use it for a motherboard or directly to the graphics card.