Except for Americans and people traveling to (or considering traveling to) and from the US, 9/11 wasn’t much of a factor in whether to travel by air in most of the rest of the world in late 2001 and early 2002. It was a drag on travel demand in various places, but it was not as much of a drag on travel demand as this virus situation.

This virus situation is very different than 9/11.

A week after 9/11, I could still fly around on all my frequent city pairs domestically and most of them internationally without massive inconvenience beyond the security screening rules changing and being inconsistent even at that. Now, flying domestically in the US by scheduled common carriers to do my usual trips is very different than it was in the latter half of September 2001 and now way more inconvenient and often not even possible without going to intermodal travel or just hiring a private plane. And flying internationally now is very inconvenient because of how thinned out the route networks have become in so many more places than happened in the weeks and months following the 9/11 hits.

This virus situation with traveling is very different than the situation of traveling in the weeks and months after the 9/11 attacks.