By your reasoning you'd claim anyone who buys a Volkswagen Golf today is buying a 40 year old car. The Golf was introduced 40 years ago and you can still get one today. Never mind it has zero components in common with the Golf from 40 years back..
XP was and is doing everything the majority of users expect from an operating system. Many of the changes since XP are not exactly improvements for many of the users. Some are, some are not.
Microsoft can stop XP support in only one way. That's when they stop taking money from government or corporations for extended support. They will need to say 'no' to the hand that feeds then. Until they do so, they are obliged to patch XP. Not just for those who pay hefty support fees, but also to tose who bought their XP new, just 4 years ago.