One of my colleagues has an old iMac (~2010), which became unusable due to a succession of OS updates making it unbearably slow
Whilst the successive OSes have got heavier, just as a point of comparison. I'm still running an early-2008 Macbook (the first metal-unibody one) and it's fine - though it did get a midlife RAM upgrade and the 5400rpm HDD has been swapped out for an SSD. That might be a bit biased since that model is a joy to work on - with a tool-free catch to get into the HDD compartment.
The 2010 iMacs aren't too hard to whip the screen off the front of and swap the HDD for an SSD. Lifting out the big panels on 27" models does give me "the fear", but we've managed it a several times without incident.
With the tumbling price of SSDs, updating the storage gives them a new lease of life.