It is a big, diverse world. And various, special use-cases are best addressed with different aspect ratios. But, to me, the point of ISO 216 adoption in place of US Letter (and other common sizes) is about having a general-purpose/common-use aspect ratio with inherent properties that help simplify things across the board for a lot of people. If it wasn't already used so much elsewhere, I'd be a "harder sell" (even to myself). But, it is used elsewhere... a lot... and has beneficial properties too. Trying to clean up the mess and simplify the chaos. Remove the unnecessary extra baggage associated with other systems.
Aside: If I was king of Sun III (previously known as Earth), my replacement for "US Legal" sized paper would be 210mm × 340mm. This maintains the width of A4, but gives it a length similar to that of US Legal. Moreover, it achieves this using the Golden Ratio. Calling it φA4 for the time being.
US Legal..: 8.5” × 14” (Before mm conversion.)
US Legal..: 216mm × 356mm —> approx. 1.647:1 ratio
A4........: 210mm × 297mm —> approx. 1.414:1 ratio
φA4.......: 210mm × 340mm —> approx. 1.619:1 ratio
Aside: Apparently the 210mm × 340mm size is used for some folded paper towels and some pastry bags already. Also, Mexico's "Government-Legal" is 216 mm × 340 mm — same height as my proposal but with a 6mm width difference. (Theirs matches US Letter/Legal width of 216mm instead of the A4 width of 210mm.)
Are you volunteering to transition Sun III's physical files cabinets, storage boxes, warehouse shelving? To reconfigure their printers, rewrite their report software etc.?
There's a lot of friction and it all takes place on the ground, unlike translating Python 2 programs to handle Unicode, there's no cloud service model that facilitates the transition.
The wonderful thing about standards is that there are so many of them to choose from. -- Grace "disputed attribution" Hopper
As King of Sun III, yes I am forcefully "volunteering" my subjects to do the work. Archives can remain as-is. Everything new shall use the new system. Doing otherwise incurs penalty. If you don't use the new system, you will get pain treatments and possible death. /humour
Nevermind folios (and quartos and ovtavos).
It's a big diverse world with room for all sorts of paper applications.
[Architectural Sizes]: https://en.wikipedia.org/wiki/Paper_size#Architectural_sizes
[folios]: https://en.wikipedia.org/wiki/Folio_%28printing%29#Size
[big diverse world]: https://en.wikipedia.org/wiki/Paper_size#Other_sizes