Ping is just example to illustrate that the problem is not technical in a way which makes it easy to understand. I'm talking about this issue for more than decade (at national level), so I nailed some patterns.

Yes, IPv4 packets and address range are too small to support IPv6, but IPv6 can encapsulate them with room to spare, so IPv6 network can address and handle IPv4 networks and hosts directly. However, this may enable two disconnected IPv4 networks to communicate via IPv6, if they are both announced at ipv6 network.

For example, my notebook is behind IPv4 NAT, but bots were able to scan and try to log into my SSH via IPv6 (miredo). Theoretically, when IPv6 will support both IPv6 and IPv4, I can expose my internal IPv4 network via IPv6, which may create security risk, same as for IPv6 native network without NAT.

This is the key problem: people don't want to expose internal networks to the public, so the feature was cut with hope that everyone will switch to IPv6 and this will no longer be a problem.

In 2004, I was lead of OPS team, so I spent some time trying to invent a way to switch our internal network to IPv6, when it will hit mainstream, and found that seamless transition is not possible at all: IPv6 requires switch, because backward compatibility is disabled, just because a corporation asked for that.