I think I read somewhere that smartphone sales are slumping across the board because fewer people are upgrading every year. This is neither good nor bad. People are just deciding that their old phone is still perfectly good and does what they want it to. In this, the interest of manufacturers and consumers are at odds: Manufacturers need to sell products to make money, and consumers have to decide how best to allocate their resources among living expenses, tools, discretionary enjoyment, and savings. Money spent on a phone is not available for a vacation. It's one of the basic contradictions of the free market: Companies have to make a profit, even if it wastes resources and hurts the nation as a whole.
Since my first computer, a Kaypro 2X, I've used my electronic gadgets until they quit working or the latest OS they can run is no longer compatible with the rest of the household or the outside world. The SE is my first smartphone and I think it should be good for another 2 to 5 years before it can no longer hold a charge or is just no longer compatible with the world. That's when I'll upgrade, either to a phone with a similar form factor, or to a gadget I have not heard of yet that serves the same basic purposes.