I've actually only "bought" one smartphone in my life, and I'm still using it today. A year or so before I retired in 2013, I swapped my latest MC-supplied Blackberry for a Samsung Galaxy S2. I kept that device after retiring and used it for a couple years.
Then, DS got a new job-supplied phone and he gave me his Google Nexus 5. That was a fantastic little device. But unfortunately, I lost it a year later.
So I bought a OnePlus 3T. It cost $399 in 2016 and had flagship specs at the time. Five years later, it still works flawlessly. And the specs are still better than most mid-level Androids that sell for $200-250 today. The one area it's lacking is the newer LTE frequency bands. So mobile data connectivity can be a little sketchy compared to my wife's Moto G6, which cost $199 about 18 months ago.
I agree that flagship benefits are minimal at best for most people. I'm not really a hard-core phone enthusiast. So I have no interest in upgrading every X years. But, like my experience with the OnePlus 3T, I think there is merit in buying a great phone for longevity. The device that has my eye right now is the Google Pixel 6 Pro at $899, mainly for the camera, mmWave 5G, and the new Tensor chip designed around AI. That sounds like a future-ready phone that may be worthy of my "second" purchase.