You're quite right mate, in that each update takes more resources - and newer OS's need much more memory - and so they can indeed make an older computer unusable.XP could run all day doing everything someone like me needs on about 250 eggs (memory) , win10 needs 250,000 eggs or it cant start up . If its old and it works turn updates off . Has nobody noticed every "upgrade" from windows sees the release of new laptops with typically much larger memory capacity so they can run all the shit nobody wants .
BUT if you put such an unpatched system near the internet, you will definitely get hacked.
The older it is, the sooner you will get hacked. Don't expect to see any signs of it, because it will probably be invisible.
If its your only system (you don't have home network/wifi/ipad/phone), and you only visit a few websites where you dont put personal information in, then you might not care, and thats fine. But you can't use it for anything trusted like online banking etc, its just too risky.