Not necessary, because if it were not true — if people spend LESS time watching TV today as people spent listening to radio in the past — that doesn’t undermine the reasoning. It’s still the case that today’s generation doesn’t listen to as much radio drama, and the author can believe that this implies they don’t exercise their imaginations as much as past generations.

Not necessary, because the concept of an entertainment becoming “more familiar” has no connection to the reasoning. The argument isn’t based on a relationship between familiarity of entertainment and less exercise of imagination. It’s based on the fact that radio drama in particular requires imagination; this is true regardless of how familiar radio drama became.

Not necessary, because the concept of what entertainment is desirable or undesirable is irrelevant. The author doesn’t take a position on whether TV is undesirable; the argument concerns whether people today exercise their imaginations less than earlier generations did.

Necessary, because if it were not true — if there is SOMETHING that fills the gap left by radio as a medium for exercising the imagination — then we cannot conclude that today’s generation of TV viewers exercises their imaginations less. For example, maybe they read more books than earlier generations did, and books could require imagination. The author must assume this isn’t true in order to reach his conclusion.

Not necessary, because even if TV does require its viewers to think about what they see, it can still not require imagination. The key distinction the author assumes between radio and TV is that radio requires its listeners to imagine elements like physical appearances and spatial relationships. TV, the author assumes, does not.