There is presumably some way, at least hypothetically, to fix the mess that User-Agent has become. (Everybody is 'Mozilla' now! And yet somehow a unique snowflake 'Mozilla' that lets you be tracked...)
But it's not quite that. It's not that a device is "mobile" instead of "stationary" that matters. It's maybe that it's a touch screen. It's maybe that it's a small screen. (With an especially uneven aspect ratio?) Maybe that it's handheld. It's maybe the combination of all of those things. But it's not really that it's "mobile" exactly. And there are, and possibly will be increasing proportions of, devices that have some but not all of these qualities we package together in our brains as 'typical mobile'