The F-35 air fleet was included as a line item in last year’s annual defense budget, but the Pentagon remains “behind schedule” in maintaining the fighter jets, the Government Accountability Office found.
The GAO report also showed the $100-million-apiece F-35s are capable of performing missions just 55% of the time, and more than 10,000 of the aircraft have been grounded for repairs. Just 450 are currently fielded.
An aircraft can be flown even if it is not fully mission capable. If a fleet is grounded, no one flies.
The F-35 fleet true goundings have been of a very short duration generally and typical of all branches flying them, to do special inspections or TCTOs when fleetwide problems are found.
The data you cited says "10,000 of the aircraft have been grounded for repairs. Just 450 are currently fielded." There is a disconnect there. The entire US military doesn't have 10,000 aircraft.
Speaking only USAF numbers, The B-1B, F-22 and VT-22 all have mission capable rates in the 50% range or less. The C-5M is dismal. Conversely the F-16 is in the very high 70%s which is considered very good. The higher the technology, the harder it is to get 100% of the systems operating on a single aircraft. This is a lesson that needs to be re-learned by those who are proponents of unmanned next gen fighters.