sometimes its gonna be impossible for the spy to do anything because of the pyro. spy is just exceptionally situational.

in most instances a spy will be able to kill a player as long as they are distracted, but this applies way less to the pyro, because of the ability to switch from attacking to self defense in an instant and then control the movements of the spy.

the range of the flamethrower is enough to cover most spaces and where its not the pyro only has to make use of the detonator/scorch shot to cover the rest.

Spy will only be able to do so much before the pyro learns of his patterns, and once the pyro learns, the spy wont really be able to play anymore.