Me and my son just upgraded his PC. It was a 4 yr old PC. We wanted new graphics card and we had to replace entire PC except power supply (barely!) and the case.
Even more fun, we wanted to build a PC from the leftover parts for friend of my son who is less fortunate. OK not top of the shelf, but he can play some games. We ended up spending $150 for new power supply, overpriced memory (DDR2), thermal grease, new fan (plastic leg broke off the original) and set of screws (where are they when you need them?).
In the end, upgrade = new PC, setting up the old one was still super expensive ($150).
Another factor is: these phones get a beating in daily use. While it may still make sense (technically) to keep some components during upgrade, they will be in bad bad shape.
It may make sense in context of repairs. Just like a car: I will fix it buying parts as needed, but after N years and M miles I will replace it.