If we had room to speculate on the wide variety of disasters that may befall the Earth within the next, say, 200 years, then perhaps hiding underground isn't the optimal best solution.
Aside: I'm sure you'll agree that it'll be easier for Elon to keep his family alive if he's adequately defended. Regardless, he might not have a choice. The third amendment was not always a thing.
As an example of the difference, consider "widespread crop failures followed by food riots causing a general breakdown of transportation networks". The social order is destroyed. But the air is still breathable. And where you live the local crops might even have been fine until pillaged by hungry people.
On a side note, the 3rd amendment has only come up in one legal case, and it was decided in that one that the violation of the 3rd amendment happened, but the government had a qualified immunity because a reasonable person would not have known of the rule. See https://en.wikipedia.org/wiki/Engblom_v._Carey for the bizarre case.
Also most collapses to worry about are not "the Earth just became immediately uninhabitable" but instead "there is a complete collapse in civil order". Think the power grid goes down like it did in 2003, but on a wider scale and doesn't come back up. As we go from inconvenience to mass starvation, what happens?
If you hide away, the survivors aren't just going to conveniently die off. They are going to remain, and WILL have a grudge.