I reiterate: What we need is for ISPs to block packets originating from their network that that don't have correct return IP addresses;
Imho, in the case of an UDP RDDoS, it seems unfeasable to me. In a nowaday common ISP, networks are very very intricated and the cost to decide for each datagram if we've a valid OIP is far too heavy, and that's maybe impossible if not dangerous with a living network.
This is more of a loss of instrument data problem.
FYI, there is always, as a backup, conventional (non electronic) flight instruments on longhauls. Always.
You are in the hall of the mountain king.