They can hit the target
No real reason not to upgrade. The key thing is not even (perhaps) the extra stuff windows 10 offers it is the fact that they are switching to a rolling upgrade cycle after that so no windows 12 in two years. Therefore if you get windows 10 you will have your pc supported until Microsoft dies whereas if you keep on XP or whatever then at some close point your pc will no longer be supported.
Now I know the haters will all shout how common users should just switch to some weird non-ms operating system but it wont happen so there is a good chance that the ecosystem Microsoft proposes will happen.
As an aside Windows 10 seems a good improvement to me and I am enjoying playing around with it. MS have taken great steps to try and improve useability and bloat which is a pleasant change from the Windows 8 'you will use tablet mode'.