Funny how people can see that differently. You think it's wasting money to own more than one device at a time.
I feel I don't get my money's worth out of the device (wasting money) until it finally dies of old age.
That's why I just sell the old device when I get a new one, or even just give it to a family member so that they can continue to use it. But if I keep a "backup" phone it's not going to do anything for me except sit in a drawer and collect dust. I don't know what I would use it for when I have a superior device that will do the same things.
An example would be my iPad Mini 4 which I felt became redundant when I got a 6th gen ipad. I managed to sell a nearly 3 year old Mini 4 for $350 CDN (about $266 USD).
Old iPhones either get sold or passed on to my mom, who between my and my brother's hand-me-downs has not actually had to buy an iPhone since the 3Gs.