> IPv6 also required hardware in the wild to be replaced

I understand that, but it was (is) able to do that in a mostly backwards compatible manner.

My question is: how is that possible with different MTU sizes? Have ISPs support 9000 byte frames and fragment to 1500 bytes for compatibility with the wider internet? Then something like PMTUD can be used to bypass this fragmentation when supported?

To be clear, I would love to have larger MTU sizes. I just don't see a straightforward way to transition everything over. I'm not a network engineer though, maybe there just isn't enough a strong enough impetus for anyone to dedicate the resources to this.