You ever ran out of ipv4 addresses on a home network? I find this probably does not apply to vast majority of users. And even if you do run out... If using DHCP is it trivial to change network prefix.
The S in IoT stands for security, of course. But it's going to be a bummer if my IoT devices forces me or my guests out of my network.
IPv6 can be made stateless with SLAAC so DHCP (and any DHCP-related limitations in the router) are completely out of the picture.