You're confusing two messages. One is that building a fallout shelter is not a good way to optimize personal safety. The other is that optimizing society safety is, for some unspecified reason, more-virtuous than optimizing personal safety.

I consider both those arguments relevant to this post. What I'm saying is that building fallout shelters is unlikely to be optimal for personal safety because there is generally much lower-hanging fruit. Further, in the event that building fallout shelters is optimal for personal safety, your efforts would be likely better spent elsewhere because pursuing personal-level solutions for society-level hazards is highly inefficient.

I omitted the obvious third argument against fallout shelters (that they increase the odds of nuclear war, albeit only slightly) because I evaluated it as likely to make people think that this post was actually about fallout shelters.

The first point is historically wrong. In the time when people in the US built fallout shelters, most people who built them thought it was more likely than not that there would be a nuclear war soon. They made the correct calculation given this assumption.

I'm not sure that that's reasonable to say. As I pointed out, personal safety is personal, and thus your decision to build a fallout shelter is subject to a wide range of confounding factors. I believe that it is likely that most people who built fallout shelters could have purchased expected years of survival for cheaper, even on a personal level. Typically fallout shelters seem extremely unlikely to actually be the lowest-hanging fruit in someone's life.

The second point is simply a referral back to a set of presumptions about ethics (selfishness is bad) that should themselves be argued over, rather than the examples here.

I assumed, perhaps wrongly, that that was a given on this site, given previous discussions here. There's probably an argument to be made that all such actions are merely purchasing fuzzies and that protecting yourself is purchasing utilons, but I'd like to think that we're better than that.

The argument that you shouldn't build a fallout shelter because the life you'd live after civilization was destroyed wouldn't really be worth living is contrary to what we know about happiness. It is a highly-suspect argument for other reasons as well.

I'm aware of the studies and arguments used to claim that happiness will reset regardless of what happens to you, but I think that a full-scale nuclear war falls outside the outside view's domain.