The second reason I don't believe it's a conspiracy comes from my experience as a software developer with 40 years experience I've not only seen this over and over again, but done it myself. It's not a conspiracy, just how the software market works. Every iteration adds new features, new features require code which makes the software bigger and more resource hungry hence slower. Visicalc, the first spreadsheet ran on Apple II computers in under 32K. Version 1 of Windows ran in under 640K. The first versions of Linux could boot from floppies and ran quite happily on my i386 computer with 4M of memory, I retired my first Android phone, a Motorola, when updates to Google Apps got too big to install without uninstalling too many other apps I considered vital.
Faster hardware with extra memory adds to the cost but more importantly uses more power & reduces battery life between charges. When a consumer phone is released the hardware including battery capacity is scaled to match the requirements of the planned software. If a consumer device like a phone (Android or Apple) is going to work well with software that won't be designed for 3 years after the release of the hardware it's going to be massively over engineered. My iPhone 5S is nearing end of life (The battery is no longer holding charge as well as it did) & when it goes I'll probably replace it with a cheap Android; if I get 3 years use out of that phone I'd consider myself extremely lucky but at the price they are I can replace it every year to 18 months and still spend less than I would on buying an Apple every 5 years.
What's in these upgrades? Some of it's bug fixes & security patches but there's a lot of adding new features to keep up with the competition plus adding extra subscription services. If it was just the fixes and patches, the phones would remain useful for longer (but not indefinitely) but good software developers are well paid so software companies are loathe to go down that path of maintaining multiple versions of the same software. I can imagine this would be specially true for consumer electronics where there is not that much of a revenue stream after the initial sale.
In any case there's a certain number of phones each year that are lost, smashed, or just fail so there is a constantly shrinking number of devices that would run that software. Batteries don't last forever either & it's always a toss-up for me between replacing the battery and buying a whole new device.
(An earlier version of this post appeared as a comment and two replies on a Facebook thread)