Well I never actually mentioned an operating system but I'm still using my 2012 iMac, but I have added 32GB of RAM and an SSD. Haven't had a laptop in years.
The majority of my friends and family who use MacBooks of various types are much the same, they keep their systems a decent length of time. Of those I know who actually use Windows systems, most aren't techies and I know a fair few who replace their systems fairly often. Certainly more than I would deem a respectable length of time to keep a system (which before you ask I'd say is no less than 3 years if you don't upgrade components.)
The most common reason they tell me is that it's become too slow. Now I know they don't buy underpowered systems in the first place, so I can only assume they're not keeping up with maintenance. Or they're overloading the system with unnecessary crap, which based on what I've seen on some of them is definitely the case, how many browser bar extensions does a person need? I'm all in favour of none, but that's just me.
These aren't the sort of people who upgrade components, nor does it seem are they the sort of people who pay heed to, get all of that crap off your computer

they prefer to just buy a completely new system when things slow down.
I've known people who'll buy a completely new laptop just because there's a new version of Windows out. Despite being advised to keep the one they have and just upgrade the bloody software. I don't know, maybe they just like to spend money