What does the lifespan of a PC have to do with the cost of Office?
None of my PCs have ever included a copy of Office. One had a demo of 'Home and Student', and one came with a three-month trial of 365, but that's it.
I don't bother with Office myself, but Office 365 costs less than keeping up with the 'offline' Office releases.
Besides, your math is off. Office 365 is $70/year if you buy it annually. The equivalent standalone product is Office Professional 2016, for $360. So that's FIVE years for the cost of 365 to catch up to standalone Office... and that doesn't include whatever it costs to upgrade to Office 2020 or whatever.