On a recent Triangle Tactical podcast,
@FlatFender mentioned a podcast he listened to where the guest was John Correia - he's the guy who runs the Active Self Protection youtube channel (great way to get a sense of what real-life self-defense situations tend to look like). Anyway, John has watched thousands of self-defense shootings and I think he said he'd never seen someone who wasn't LEO go for a spare mag (Luke, pls correct me if I'm misquoting, and it was "very rarely" instead of "never").
I carry a spare for reasons others have already articulated in this thread (plus it's a G42, so I'm not working with a lot of rounds to begin with), but I thought that was an interesting data point from someone who is an expert on how self-defense shootings go down.
Also, I don't think John has any video of a terrorist in action. I think we'd all want to make sure that someone who is planning on suiciding themselves and murdering everyone around them is incapable of further movement (including reaching for a detonator).