When an airframe is stressed beyond its design limits it can't be put back into service. It's conceivable that it could be put through a "C-check" type process of disassembly and inspection, but given the age and residual value of the plane that might not have been cost effective.

Planes are very fragile and are not meant to be able to sustain loss of control at high speeds. Sudden disintegration does happen mid air following a loss of control.

I am always impressed at how they look like a big chunk of metal from the outside, but they are in fact mostly made out of air and very thin and light materials.

As an uneducated guess, the wing mounts (aluminum?) and surrounding fuselage have stretched way beyond tolerance and are no longer capable of supporting nominal load. The metal has fatigued and must be replaced, which would require new wings and nearby fuselage -- at no small cost.

I'd imagine it's less a case of the plane being damaged heavily and more a case of there's just not enough value gained by keeping it in service to offset the potentially disastrous cost if something were to go wrong.

The article said the plane had been subject to very intense g forces, so I would imagine substantial structural damage. They should feel lucky that it didn't fall apart.