The other things you're naming aren't L3. But they're still different for v6 because of the split network.
There's a lot of other stuff that makes it not a seamless cutover. Randomly-assigned addressed, ULAs, no NAT by default, and all the other v6 extensions.
What they could've done (and maybe will do) instead is just make a new thing with a larger address field but keep the rest the same.
Making a new thing that changes nothing but the address field size won't give you a seamless cutover, because there's no seamless way to use addresses bigger than 32 bits with v4. (If there was then we wouldn't have bothered with v6 in the first place, we would have just used whatever that method was!)
Nothing much else has really changed in v6 -- v4 has ULAs and doesn't use NAT by default either, just when under address space exhaustion. The randomized addresses are a thing, but making a new protocol without them isn't going to produce something that's any easier to switch to, nor any more compatible with v4, than v6 already is.
Meanwhile, DNS and such would need to be upgraded to support 128bit addrs, but they'd still work with the old ones too, so again easy decision. Then once it's safe, any ISP short on addresses can start handing out /64s.
I know there are also v4 to v6 maps, but that's not default and is forever limited to 32-bit.
But you understand that after ISPs get short on addresses and start to hand out /64s, it's not going to be exactly the same afterwards, right? You'll have to actually use that /64 and the updated DNS and stuff.
Because we hit that point twenty years ago. We're long past the "everything looks and works in exactly the same way (so only the 32-bits of addresses in v4 work)" stage and deep into the "ISPs hand out /64s" stage. There was a point where v6 deployment just meant that you turned it on and nothing else at all changed, but at this late point in the game it also involves using those new addresses. We already took the approach you're asking for here, we just didn't stop at the beginning of it.
I'm too young to remember, but I've dealt with old routers and PCs, don't recall that ever being the default. If it was, they took the second step too early.
Btw, there's also a difference between dividing up the existing blocks vs handing out new ones.