The problem is, it is NOT a simple idea, but one that requires a lot of relative heavy lifting with deep packet inspection for a lot of protocols that have to deal with the spaghetti that is NAT.
And quite honestly the default configurations for IPv6 on consumer routers is "wide the fuck open"--which is not at all what I want. But if they don't make it "wide the fuck open" suddenly you are asking normal people to learn how to punch holes in their firewall.
The default config for most routers is to deny inbound connections, for both v4 and v6. I'm certain you can find routers that don't do that, but it's not common.