I am not this kind of network engineer, HOWEVER. Both IPv4 and IPv6 are versioned by the first 4 bits of the packet. Depending on that value the address size and location are fixed.
I would not consider the comparison of the source address of packets crossing an ingress link to be 'deep'. I consider that check to be very shallow. It needn't even be every packet from a set, merely picking a random (actually random) packet and testing for conformity is a good quality control measure that SHOULD be taken.
What would the comparison be against? Routers are supposed to know which links are on the other side of all down-stream connections so that they can effectively route.
Why would your ISP allow you to send packets with a source address it hasn't allocated to YOU? That kind of check/enforcement is pretty cheap and simple.
To impose fixes upstream, you'd have to do DPI on all data; which is not allowed under some laws (i.e. net neutrality).