|Ammonia Tank in Shuttle
Here's the gory details: should the cooling system ever spring a leak, delivering the 1,800 pound replacement ammonia tank (ATA) would almost certainly require the capabilities of the space shuttle. The weight is OK for Progress, but not the size. Other system failures can only be supplied by shuttle missions, such as replacing the huge solar or cooling radiator panels. Please read the linked article about micrometeorid damage to the radiator panels:
The astronauts and mission control want to send the 780 pound pump back to Earth so it can be inspected to find out why it failed. But the final two shuttles are totally filled up for the return trip and won't be able to return it. Progress can't return it; the shuttle is the only way to return the pump.
What else could fail on the ISS which requires the shuttles to deliver or return? Probably a dangerously long list of items.
That's the reality of it. Scrap the shuttles and you spin the roulette wheel that some failure won't happen which can only be fixed using the resources of the shuttles and their cargo/return capability.
Congress: Save the Shuttles. Cancel the layoff notices immediately. Resume building the fuel tanks and solid boosters. Fly the shuttles at some minimal rate to ensure their continued launch capability until new heavy lift is available. Or wait until a preventable failure destroys America's 100 billion dollar investment in our future. That would be a crime of unbelievable magnitude.