If there is no circuit concept in the IPv4 and IPv6, then the value of the field could (in principle) vary wildly from packet to packet. In real life condition

If there is no circuit concept in the IPv4 and IPv6, then the value of the field could (in principle) vary wildly from packet to packet. In real life condition it most likely wouldn't - and even if it would, maybe that would be actually better than transparently fragmenting packets just because MTU changed after higher level connection has been established. IPv6 packet header is significantly simplified when compared to IPv4. Although it is twice as large, almost all non-essential fields have been moved to extension headers. I guess it wouldn't hurt too much if there was an optional header with PMTU updated by all supporting routers along the path, especially when there is no checksum field in IPv6 packet.