I have heard many replace their phones due to dropping them and becoming unusable. But everyone uses a case now and the build quality is generally better that one mishap does trash the phone. Most people I know getting new phones now did so bc their old phone "got too slow to be usable." I believe that's a matter of new OS versions really are much heavier. Both my last 2 phones I had upgraded bc I went one version too far and had a nearly bricked phone.
Any phone that gets more support than it should have, such that the only OS you can install is too slow to make using the device enjoyable, makes it more likely for the device owner to throw that device out, and then it becomes e-waste.
It also harms software preservation. Sure, we have IPSWs for every single public build of iOS that exists (and if you dig around, probably a ton of betas and even internal builds). But you can't really do anything with any of them once you get to the point in the iOS product line where things were sufficiently hardened