Just my .02 cents worth here.
I'm a mechanic, although on vehicles,still similar problems in business.
We frequently replace parts that are still working just fine,not trying to rip anyone off at all.
What it comes down to is the expected service life of the component,the estimate of the life remaining,
the consequences of that part failing,the warranty that we have to provide on that part and who pays our time if that part fails after we fit it.
In a perfect world we could look at that part from 20 feet away and say it will/will not fail in the next 29.5 days.
One thing clients don't tolerate is telling us to check something,have us say it's OK and then it fails half way though their holidays.Yes we saved the price of a pump or whatever it is,but in the big picture we stuffed their holiday.
So we as mechanics have to be very conservative.When it's your own boat you can do what you like,we can't.
Then there is the question of labor charges,is it worth spending an hours labour on an old pump? Better to put the $ toward a new one.
Sorry for the rant,rarely do customers ever say we rip them off,
most mechanics I know put their customers best interest first.