I thought I heard that each orbiter had a life expectancy of 100 flights. With a 5 vessel fleet and approx 5 launches per year (107 launches in 20 years), the Space Shuttle program was good for 100 years...or until something better comes along.
You and I have heard a lot of things.
The actual catastrophic failure rate is 1/50. With four orbiters, that means 200 trips and it's over.
And the losses are of course losses of irreplaceable heroes and explorers, not merely billions of dollars of steel and silicon.
The mission concepts and the lack of alternatives or followon vehicles are simply not consistent with a failure rate of 1:50.