Manufacturers usually maintain a phone (as far as issuing updates) for about a year, then stop writing updates for it. We can usually still use them well enough for about another year. Then it's time to get a new phone.
It's not the way it should work - it's just the way it does work.
Remember - Microsoft and Apple don't give us free system updates for any length of time - if you buy a Windows 8.1 laptop tomorrow, and Windows 8.2 comes out in March, you stay with 8.1 or buy the update to 8.2.
If you buy a popular phone, people usually write ROMs (updates to later versions of Android) for it for 2 or 3 update cycles. (And they're free, even if putting one in voids the warranty - which is gone after the first year anyway.)