Another perspective on this is … if you don’t need the update, why would you upgrade?
The real answer to the question would then be … you upgrade when you are forced to because the older spec of technology simply doesn’t work any more. So if the financial or environmental considerations are greater than your desire for the latest and greatest technology, I expect the phone to function for many years.
Every phone is limited in how much can be upgraded “in place”. The Librem 5 is better than most.