Saving stored planes for back up and relief operations

I was recently watching some YouTube videos and reading of incidents involving PIA flights being stranded at unscheduled stops due to technical problems.
Not only was the plane unable to continue the flights due to these technical problems but the passengers on the flight were stranded along with the crew. Passengers expected to fly on the plane on the return journey on the plane back to Pakistan were also delayed for hours.

PIA was forced to send another plane with a relief crew to repaid the faulty plane and to pick the stranded passengers, only adding to financial losses by the airline.

I know it doesn't happen that often that a plane is stranded due to technical or other problems and is forced to suspend it's flight, but when it does happen the consequences can be devastating.

Using the PIA case as an example, it got me thinking why airlines don't used stored planes as backups in case a plane is stranded far away. Take for example Gulf Air's recent storage of their B767-300s to save maintainance costs. If I'm not mistaken, Gulf Air's route network is small so it did not need it's B767-300s.

But suppose Gulf Air had an incident of stranding the plane. A stored B767-300 of theirs could be dispatched to transport the relief crew and pick stranded passengers without diverting planes already in use and scheduled for other flights.

PIA also had temporarily grounded it's B747s to save fuel costs. While being grounded, the 747s should still have been equipped and left prepared to meet any emergency needs in order to minimize disruption.

PIA has a crew of experienced captains and crew members to operate B747s and to keep crew members of grounded aircraft types up to date, airlines should use flight simulators.

I've also read unconfirmed comments from various individuals that Qatar Airways plans on selling it's Airbus 340-600s as soon as it inducts the Airbus 380. If Qatar Airways was wise enough it would keep a few Airbus 340-600 for emergency situations.

This will help during unexpected emergency situations no matter how rarely they occur.