They are sunsetting future OS feature updates for it, not the hardware itself. Plus they release security patches for some older devices as well. Eg iPhone 8 supports latest OS, but they just released a patch for OS that supports iPhone 5s, which came out 9 years ago.
You make a phone. After one year, you release a newer, faster phone, and then repeat every year following. At which point do you stop producing the first phone you made?
Perhaps you say "when it stops working". Well if that were the bar then we would still be producing computers from the 90s. Ok so maybe you say "no, when it stops running the apps I want to run". Sounds reasonable enough. So when should the app makers stop supporting the older hardware? Presumably at the point when maintaining the old hardware starts costs more than they make from the support.
This point will be reached sooner or later, and you might disagree with it because you still see the product as fully capable, but ultimately the phone will stop making economic sense at some point and production will stop.