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> Yes, but let's not pretend that IPv6 was just IPv4 with a larger addr size. A lot of other unnecessary cruft got changed.

Mostly removed (e.g. removing fragmentation). If you're making an incompatible protocol, might as well take the chance to remove the cruft.

> Add an extra octet to IPv4, call it IPv5. Done.

Have fun debugging nondeterministic routing loops lol.

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Fragmentation is mostly removed on IPv4 as well. Almost everything sends DF, and there's very little legitimate fragmented traffic. You can just drop fragments wherever convenient. If you want to be nice, it's a good idea to have a small reassembly buffer, but if it's too much work, just drop them.

It's beyond too late to change anything, but I imagine if IPv6 had started with a sensible rollout plan and worked backward from there, it would look somewhat different and it would have taken a lot less time to take hold.

IPv6 doesn't remove fragmentation. IPv6 fragmentation works differently. See https://www.rfc-editor.org/rfc/rfc8200#section-4.5

The Fragment header is used by an IPv6 source to send a packet larger than would fit in the path MTU to its destination. (Note: unlike IPv4, fragmentation in IPv6 is performed only by source nodes, not by routers along a packet's delivery path -- see Section 5.)

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