There are two ways for me to interpret "simply tell your system to start using v6".
If it means upgrading every program, then your plan works but it's the same as how things work today. You're telling people to do a thing, and they aren't bothering. The "simple" step isn't simple at all.
If it doesn't mean upgrading every program, then your rollout fails on the last step. You start handing out longer addresses and legacy programs can't access them.
It's the second one. But legacy programs did get upgraded, so I don't see why they wouldn't under this other plan. If anything, it's easier because you're only making the address field bigger and it's not a separate case. Some routers struggled with 128-bit addrs due to memory, and could've gotten away with like 48 or 64 bits if they're using DHCP.
Lots of legacy programs, and current programs, and other things that could have been upgraded did not get upgraded. Getting to the situation where you can just flick a switch is not a realistic dream. There's not enough motivation for the average business to add support for a version that isn't in use yet.
If it means upgrading every program, then your plan works but it's the same as how things work today. You're telling people to do a thing, and they aren't bothering. The "simple" step isn't simple at all.
If it doesn't mean upgrading every program, then your rollout fails on the last step. You start handing out longer addresses and legacy programs can't access them.