" To go from Android 4 to Android 5, you need more memory, RAM and storage"
My Sony Xperia has gone from 4.4 to 5.0 to 5.1 to 6.0 since I owned it. It runs exceptionally well, never any slowdown, and works as well as the day I bought it...
But then I bought a premium Android device, not a bargain basement device. You pays your money and takes your choice. You can buy a cheap phone and replace it every 12-18 momths, give it to a granson or burn it, or pay more and keep it longer. Choice is a great thing. Just don't expect that £80 phone to be upgraded past the OS it has, it will only get security updates.