So you are not talking about attacking old code at all, but networking stacks that are indeed actively developed? That feels like a very different ball game from attacking Win98, even if the platform they are running on top of is old.
For anyone saying these aren't targets, no they are probably already hacked. These are the things that keep the national security folks up at night knowing an adversary has them already backdoored and set up for take down. Moreover if they execute on that they would go for maximum damage first to either create chaos, or prevent the system from being repaired easily.