Understand, of course that all of this is just one persons opinion. Take it or not.
I will repeat something you obviously missed.
This is why using XP today is dangerous. Not just for you, but your computer can be taken over and used as an attack vector for others, making you part of the problem overall. Often times people don't even know their systems have been compromised in these ways, and they are acting as an attack vector without even knowing it.
Maybe you don't understand this. What it means is that if your computer is compromised, it can be used in the commission of another attack on another computer. This is how internet worms work, moving from system to system infecting everything they can find and using them as new bases of operation.
It's like when someone uses someone elses open wifi router to commit a crime. The police will come to YOU, and you might claim "Oh, well, someone must have been using my open router", well that may be hard to prove and more importantly at some point YOU have to have responsibility for allowing that to happen. The same is true for open email relays, which are used for Spam. The internet rightly cuts these off, and is the primary reason most ISP's won't allow you to run your own email servers anymore on home networks.
When you put your vulnerable computer on the internet, you are becoming part of the problem of insecurity, and becoming a piece in the attack infrastructure of criminals. THAT is why I care what you do. Your actions affect me, and everyone else who are citizens of the network.
If you want to run your 15 year old OS in isolation without connecting to the net, by all means... I won't argue.