Depends on how you feel about security loopholes because smart phones won't get updates forever.

Smart phones are in no way BIFL.

Get a somewhat popular/high volume phone with an unlockable bootloader. You'd be surprised how long some phones continue to receive security updates through 3rd party Android projects, even after they are obsoleted by their manufacturer.

Check: https://wiki.lineageos.org/devices/

Bonus if you get a highly repairable phone which is unlockable and well supported by third party Android projects.

Let me tell you my story of a closely-related mobile device and its relative longevity.
I bought a first-gen iPad Pro, the 12.9” model, as an open box from a reputable online retailer (not Amazon) in 1/2016. I use it semi-professionally, and the battery finally failed a year ago. I called my local Apple Store in a panic, and I took it in. The warranty was long expired, but they gave me the option of a battery replacement for $100… I took it. The shocker was, they didn’t actually replace the battery… they had a mint-condition refurb in the back, of exactly the same hardware config, even down to the correct color! They managed to wipe my old device after transferring everything “mission critical”… I had a local backup for everything else at home. The refurb has been perfect ever since, and I’ll keep it until they end security updates (no end in sight). Again, all that for a flat fee of $100. No blemishes, absolutely as-new. This as close to BIFL I’ve ever gotten with a complex battery-powered electronic device.

As long as you can replace the battery in a smartphone you should be good for a long time. It's the battery that always seems to die before anything else, at least in my experience. And modern lithium ion batteries seem to bloat more then older lithium ion batteries in my experience. I still have my old GBA SP battery in my SP and although the battery isn't the same capacity as it once was it hasn't bloated like every modern phone battery or mobile device seems to do these days. I assume it's the increased capacity and density?

Obviously try not to drop you're phone on the floor all the time and it will last a good while.

I'm still using a Motorola RAZR HD bought in 2013 - with the original battery still giving me more than a day's use.

But I only use it regularly for phone calls and SMS - occasional email and web browsing, no games, no social media.

Blackview BV9900/9800. It's an armoured phone that runs on Android, made for industrial & outdoorsy environments. If you get the phone case/screen protector too it'll basically be indestructible. I've had mine for >5years, dropped it on the ground numerous times, hard tarmac, concrete, stood on it. Still going strong and the screen is unaffected. The downside is the weight, most of it is a massive battery that lasts about 2 weeks. The camera isn't great, dropping it on your foot hurts, but if you're a big bloke not an instagram model it shouldn't matter.

My OnePlus 3 and 7 have lasted longer than previous Huawei, HTC, LG, or Motorola phones. I refuse to buy Samsung or Apple out of stubbornness.

Lithium batteries are good for a thousand charges. Thats when a mobile device starts to fade. I think in some models one can replace the battery. Even Apple has issued a repair manual, but its not easy.

Your best bet is to get a phone compatible with custom roms. Like lineageos, calyxos, grapheneos, etc. The Google Pixels tends to be the few modern phones that still support a custom rom, without being a phone from 5 years ago. Unlockable bootloader, and a pretty repairable phone if the battery dies or if the screen breaks. Make sure it's an unlocked phone, otherwise the sim card company won't allow you to use custom roms.

Buy it for life VCR

Phone companies intentionally slow down older phone models with software updates, I remember reading recently that some smart phone company has a hidden setting where you can switch for your phone to save your cpu or battery life as it ages. You can get a sturdy smart phone but it will be slow as shit eventually and unbearable.

Check out the fairphone: https://www.fairphone.com/en/ Don't have one myself but my Dad has and loves it. Uses a standard Android OS but it's designed to be repairable and upgradeable. Definitely not the cheapest but they are literally designed to be BIFL so worth considering. There is another company doing a similar thing but the name escapes me rn.