Not in every country/region, unfortunately. In my case it's cheaper to upgrade my phone and sell my previous phone than get a BYOB device by a significant margin.
Yes. Though in terms of physical waste that the original commenter brought up, selling your old phone for someone else to use makes that upgrade a pretty good option.
We could look at the total amount of years any phone is used and abstract away from who uses it. Slightly stylised, if we have four people, Alice, Bob, Charles and Dave, and Alice gets a new phone every year, Bob buys Alice's old phone, Charles buys Bob's old phone, and Dave gets Charles' old phone, then everyone changes phones every year, but each phone is still used for a full four years.
