[throw0101a]

>> In other words: The current IPv6 specifications don't allow public IPv6 addresses to send packets to public IPv4 addresses.

> Why didn't they? What are the arguments against this solution?

They did. See "Stateless IP/ICMP Translation Algorithm (SIIT)"

   This document specifies a transition mechanism algorithm in addition
   to the mechanisms already specified in [TRANS-MECH].  The algorithm
   translates between IPv4 and IPv6 packet headers (including ICMP
   headers) in separate translator "boxes" in the network without
   requiring any per-connection state in those "boxes".  This new
   algorithm can be used as part of a solution that allows IPv6 hosts,
   which do not have a permanently assigned IPv4 addresses, to
   communicate with IPv4-only hosts.  The document neither specifies
   address assignment nor routing to and from the IPv6 hosts when they
   communicate with the IPv4-only hosts.

* https://datatracker.ietf.org/doc/html/rfc2765 (2000)

* https://datatracker.ietf.org/doc/html/rfc7915 (update in 2016)