Your arguments are not really responsive.
A big rolling suitcase full of powerful explosives and shrapnel set off in a security line at peak time in a large airport would be way worse.
I gave you an example of a car bomb that killed only 30 people even though it was set off in a crowded market. Have you ever seen a car bomb go off? I have, it's huge. What's your basis for assuming that a suitcase bomb is going to be so much more devastating?
Certainly there is plenty of opportunity, but you're making a chicken-and-egg argument by saying there's very little terrorism, therefore security is a waste of time. I'm saying that that the payoff for the risk involved is not enough for most people.
And if that's not enough, do it again, and again. One security line bombing a month until the end of time would be fairly easy and would cause complete chaos.
I mentioned the northern Irish terrorist problem because I'm from Ireland and later lived in London. One bombing a month does not cause complete chaos, it just pisses people off and creates more public support for stiffer security measures, more intrusive surveillance and so on.
I suggest you step back from your assumptions of what would happen and look at available documentation of what actually does happen in countries with long-running insurgencies or terrorist problems, from the UK to Sri Lanka to Colombia, cases of actual disasters (whether engineered or accidental) at airports and public transit hubs.