That said: for every idiot who hooks up a Windows 98 machine to the internet, there might be some other idiot checking whether exploits targeting it, still work. Or exploit kits that sniff an OS, and select exploits to apply accordingly.
Vulnerabilities tend to have a long tail...
Probing for obscure OS that isn't there on the other end = opportunity cost.
Both probably. I don't see where the "opportunity cost" is when you can trivially do both. Please describe the opportunity cost in detail; that is what I am asking about.