> not enough v4 IPs

No one is giving their IoT devices public IPv4 addresses. They would be behind a NAT. RFC 1918 provides 17,891,322 usable IP addresses for each private network. If we want to be a little more adventurous, RFC 6598 provides an additional 4,194,302 usable addresses and 240.0.0.0/4 is another 268,435,454 usable addresses "reserved for future use" since 1989, but still sitting unused so we can use them as internal addresses inside a NAT anyway (for example, AWS uses this range internally).

Show me a network that is using all 290,521,078 addresses and I'll show you a network managed by a team of network engineers who can just set up IPv6.