Apple uses terms like vintage (5+ years) and obsolete (7+ years) versus end of life.
They have nice list here:
from apple site:
Vintage and obsolete products
Owners of iPad, iPhone, iPod or Mac products may obtain service and parts from Apple or Apple service providers for 5 years after the product is no longer manufactured (or longer where required by law). Apple has discontinued support for certain technologically obsolete and vintage products.
Vintage products are those that have not been manufactured for more than five and less than seven years ago.
For me its
can I install the latest OS or not?
can you still get parts OEM? Can I add enough ram or an ssd? do they have the ports they need?
can people do what they need to do with out affecting real measured output productivity? Often people say oh yeah it's faster, but if you get more work done, it's not always an easy answer.
how often do users reboot, once a day, week or longer?
if only once a week or longer... is the new mac "significantly" faster after it's booted and apps are open and cached?
other costs are how much IT time does it take to support MAC vs PC users and systems?
when looking at MAC vs PC outside of cost/performance , another factor most ignore is apple scripting. It's like having excel VBA, but across any program.
Google will make people write a business case if they want a PC, I don't know why, I know google has their own custom IT apple tools that we don't but there have to be more complex reasons why a more expensive computer has an overall lower operational cost for them.