vfat only won in the sense that if you're forced to use windows you still have to use vfat. In every other context there is a better option. I haven't needed to move files to a windows box in 15 years so—to me—it's a massive surprise anyone still uses it at all.
Just don't use USB keys; they're terrible security-wise and you're forced to use technology built for your grandmother.
That is the only sense in which a 'universal file system' can win. There is no other context. Your personal usage habits are completely irrelevant ("I haven't had to shoot a gun since the first gulf war, so to me it's a massive surprise that anyone still fights wars at all.")
I don't use USB keys. I don't have any windows systems. But I do have UEFI, and that is based on FAT as well. Disregarding my grandmother's technical prowess, this technology is here to stay, and sticking your head in the sand about it doesn't advance any causes.
I live in an environment where I need to move files between several builds of Windows, Linux and macOS - and often where there is no network access because of security policy (misguided policy perhaps). Thumb Drives are the only way to function.
Smells like the IT department of BigCo or public agencies to me. Been there, seen that. Literally terabytes of Debian binary packages transferred by hand with USB keys and disks between the dev network and the secure network.
Oh, and the USB serial numbers of the keys had to be whitelisted on the secure network. What a fun that one was. They didn't even stop doing that after I showed them that I could modify the firmware of some of the sticks using software from some obscure Russian site and thus bypass the control...
(Does anyone know by chance how that website is/was named? All I remember is that it held hundreds of manufacturer tools for all kinds of USB stick controllers/flash chips)
I work in the public service industry - many of the systems I work on are isolated from the internet, and from other networks even - so I often have to sneakernet software updates and other things around to all these various machines. The no network thing is reasonable when you consider the requirements these systems operate under - its also an easy way for folks with no system administration abilities to ensure some security and stability.
Just don't use USB keys; they're terrible security-wise and you're forced to use technology built for your grandmother.