Not necessary, because even if visitors today do NOT generally look at more pieces of art during a visit, that’s consistent with the author’s reasoning. In fact, it might even help the argument to learn that people don’t look at more pieces of art, because that might constitute additional evidence that people are less engaged with works of art than they once were.

Not necessary, because what allows people to go through museums faster is irrelevant. What matters is whether the time spent looking at an artwork in a museum is evidence of level of engagement.

Not necessary, because “enjoyment” is irrelevant. What matters is whether the time spent looking at an artwork in a museum is evidence of level of engagement.

Not necessary, because even if the visitors who take snapshots of works frequently look at the pictures afterward, the author’s reasoning is based on the time spent looking at the work of art in the museum. The author does not have to believe time spent looking at a picture of a work of art is equivalent to time spent looking at the art itself in a museum for the purpose of determining level of engagement. In addition, we only know that people “perhaps take a snapshot” — so we have no reason to think that the people who take snapshots represent a significant portion of museum visitors generally.

Necessary, because if it were not true — if the amount of time spent looking at a work of art is NOT a reliable measure of engagement with that work — then the fact people today tend to spend under a minute looking at artwork would not constitute evidence of their level of engagement with artwork.