This is probably the part of it and it probably used to have stronger effect, but right now I think the reason people change smartphones often is the same reason people changed their PCs every two-three years - software increases in features / bloat so fast that if you don't upgrade, you'll quickly be out of the loop (and God forbid you actually update your software - then it will gradually stop working as authors make it bigger and slower).
It's all about life cycle. A typical car lives for nearly 15 years. There are huge numbers of 15-20 year-old cars on the road today. An introduction of a new type of car won't change the life cycle dynamics. Even if auto manufacturers manage to sell 50% of new electric cars in 10 years, there will be tens of millions of old gasoline cars on the road ... just because people keep driving them.
It's a big part of why Apple makes such incredible amounts of money from iPhones, but their iPad revenue has dropped off significant.