It's a little more nuanced than you're making out. I spent way, way too long working out the totals from the various methods of getting a new phone and getting the free phone as part of a 24/36 month agreement ended up being cheaper than many alternatives, primarily because you're paying the monthly plan amount whether you take the free phone or not. I personally think upgrading my phone after three years is a reasonable timeframe, but of course everyone is different.
It wasn't cheaper than all alternatives. There were a bunch of virtual operators offering better monthly rates than the big networks but I've personally had bad experiences with network deprioritization on them. Depends very much on your individual circumstance, I'm in NYC and the network is clearly pretty saturated.