the internet has a whole bunch of non ethernet stuff, a lot of which has different frame sizes. Its totally possible that backhaul is running jumbo frames, or something like it, but you'd never really know that.
Conversely ADSL has odd frame sizes(inherited from ATM; 48 bytes if I remember correctly), but you don't see that because its hidden from you. Cable has a frame sizes ranging from ~500 up to 2000 bytes. Again, hidden from you.
One of the joys of TCP/IP is that different frame sizes are handled for you. Sure it might be beneficial to have a frame size that marries up with packet size, it might not. you don't really know, because the internets.
The bottom line is that if you ever send an IP frame larger than 1500 bytes outside of your own network, it's most likely that it will never reach its destination. Especially for IPv6.
I wouldn't call them "frame sizes" for ATM or DSL. It's a bit of a transparent fragmentation into "cells" onto the "transport" layer. It could carry arbitrary "frame" sizes on that
It being a thing on "the internet" would require it being a thing on a sizable majority of the internet. You'd need to get large network operators, peering points, user-facing ISPs, and even users themselves on board to change their setups.
And Path MTU discovery is still sufficiently unreliable as to make it incredibly painful to have partial large-MTU networks.
And if you do any of this with standard home customers, a hellscape torrent of user complaints is going to rain down on your support contacts. Which costs money. More money than is lost by the higher cost of routing smaller packets.
So, no, jumbo frames could not be a thing on the internet. There's a reason it's called fossilization. There is no technical reason precluding changing this, it's just frozen into way too many places to be changed.
This the same argument why IPv6 can't be a thing on the internet; I agree that the book isn't closed entirely on that yet, but very significant progress has been made.
Except disruptions (i.e. worse service than IPv4 only) from rolling out IPv6 are the exception while disruptions from rolling out jumbo frames are absolutely the norm.
