But why couldn't we just put a middlebox in front of the host which translates a 4-over-6 address to a plain ipv4? The host box still sees only v4 but v6-only devices can still connect to it.
Like I could envision a simple router-like device doing this.
That's not the direction that's the problem - like I mentioned (IPv6 is backwards compatible), NAT64 and so forth boxes exist (but still not great since you have to hold state).
But the problem is that the inverse is not possible. How would a IPv4-only box connect to a non-IPv4 box? How do you encode more than 32-bit of information into the 32-bit destination address field of the IPv4 packet?
Like I could envision a simple router-like device doing this.