To impose fixes upstream, you'd have to do DPI on all data; which is not allowed under some laws (i.e. net neutrality).
RFC2827, which should fix the problem where SSDP can be used for DDoS, was published in 2000: https://tools.ietf.org/html/rfc2827
Is ingress filtering on layer 3 considered DPI?
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.