I always thought as cell phones became more widely used the prices would come down, but that doesn't seem to be happening. At least not with the big providers.
Most people today, especially those under 60, have cell phones. And I do think the the prices for comparable phones have come down over time. But at the same time, cell phone manufacturers are constantly improving their products, and adding functionality. And they are able to charge more for a new phone because it can do more than a simple one-for-one replacement phone could do. My first cell phone could only be used for spoken communication - no texting, email, calculator, wallet with 'contactless' credit cards, web searching, photography, navigation, TV, etc. My current cell phone has all those functions, and also counts the number of steps I take when I mow the lawn, and serves as the clock radio that wakes me up each morning.
The other confusing factor here is that the way we pay for cell phones has also changed. When I got my first cell phone (back in 1999), the phone came as part of a contract. There was a monthly base charge plus a usage charge (minutes). Wife and I purchased our current phones outright, unlimited minutes are included in the base monthly fee, and the only variable is the adder if we exceed the monthly data usage allocation (which we don't).
But there is another consideration here - most of us are old enough to remember the days when 'the telephone company' was a regulated monopoly, the telephone itself was provided by 'the telephone company' and lasted forever. Or at least until we decided that we no longer wanted the chunky old black phone and preferred one of the newer models that featured a sleek design and colored plastic housing. In those days, the telephone system was its own power source, and the telephone on the kitchen counter was a totally passive device. So it was very possible to use the same phone for decades. Cell phones are fundamentally different in that they aren't passive, and instead must contain a power source - the battery. It is theoretically possible for cell phones to be designed to use replaceable batteries, but considerations of weight and energy density have led phone manufacturers to opt instead for built-in rechargeable batteries. All batteries have finite lives, even rechargeable batteries which have a finite number of recharge cycles. In the early days of cell phones, it was very possible to replace the battery in your cell phone when it reached the end of its life. And at least in theory, that is still possible except for the fact that manufacturers today choose to design their phones around specific batteries, and after a period of time, original-equipment batteries may no longer be commercially available, and aftermarket replacements may not be as good. As a result, when the battery ages to the point where it will no longer hold a charge, the owner of the phone most likely will opt to purchase a new phone to get the sexy new features rather than replace the battery and live with the old phone for a few more years. There is data that shows that the average life span of a cell phone battery is 3-4 years; Apple claim that the average iPhone is owned for a bit more than three years.
In those old days, 'the telephone company' was vertically integrated and had a subsidiary (Western Electric) that manufactured telephones. Today, telephones are manufactured by one group of companies, while another group of companies provides phone service. Phone service is an ongoing service business, but the business of manufacturing phones relies on users periodically buying replacement phones. If cell phones lasted forever, the market would eventually saturate and the manufacturers would go out of business. I wouldn't go so far as to suggest that cell phones are designed with obsolescence built in (although there are 'throwaway' phones that certainly aren't designed to last a long time), but the combination of technology innovations and finite battery life effectively force periodic renewal.