Re: How many Shuttles could have been kept operative..
Why are you blaming Trump for Obama's decision to pull the plug on the Shuttle Program ?
The decision to end the Shuttle programme was taken by George W Bush in 2004, not Obama. To extend the working life of the orbiters would have required dismantling and rebuilding from the ground up and re certifying, and this was prohibitively expensive. The plan was that the Shuttle would be used to complete construction of the ISS and then would be retired.
Obama actually extended the life of the Shuttle programme and authorised two additional missions.