We should be able to keep our devices until they die, but without regular software updates, you're playing with fire.
Now I do have a 10 year old Core 2 Duo 2.66Ghz with 4GB RAM, Gigabit Ethernet and a nice 1920x1200 display serving as a Plex Server. In day to day use, the only time I can tell the difference between that and my modern laptop is when I try to run too many things at once and that could be alleviated by upgrading the RAM to 8GB.
As far as the iPad 1st generation, it's true that you can't get software updates -- I have one too -- but you can still download the "last compatible version" of apps for it. I reset mine last year and re-downloaded and ran Netflix, Hulu, Crackle, Google Drive (for reading PDFs), Spotify, Plex, and all of Apple's iWork Suite.
It still supports AirPlay and prints to all of my printers. I can check my Exchange email at work. The browser on the other hand crashes constantly.
The upgrade cycle has really just been a process of filling up the market. In fact I’d argue there are still a lot of old devices still in use beyond an ideal lifespan. There’s still going to be a much quicker refresh cycle than desktops for a while, but they’re likely to converge gradually.
What we might see is high end users keeping their flagship for longer, and refresh phones ‘downstream’ in the family to more recent midrange devices rather than passing on their own device and getting a new flagship. In fact, that’s what I just did for my teenage daughters.