Well built phones can easily last 5+ years. I’ve been using my OP5T for 6 years now. It was my main for 5+ years. It is not slow by any means, even now. In fact, for most normal users, it would still seem fast enough for daily use. Camera and battery would be the only issues. Similarly, if the battery of P8P lasts (or swap it out) and if they release proper software updates that don’t slow down the device, it can absolutely last 7 years. It might become a second phone for them, or their kids can use it.
If you don’t mind my asking, wouldn’t a laptop do much better in these scenarios? (Or were you talking about occasionally using Dex for these purposes)