They have to fight with the aircraft they have.
Of course.
But any nation in that situation is sacrificing their pilots, and the quality will tend to decrease. We saw this in WWII very clearly. If Ukraine acquires “better planes and helicopters” that will be nice for them — but those will be piloted by greener kids who are easy to blow out of the sky.
You lose your good pilots when you send them up in bad planes.
You lose your good planes when you send them up with bad pilots.
They don’t have the option of sending good pilots up in good planes. So they do what they can, but the outcome won’t be pretty.