It is in no way irrational to prefer a plane with an established track record over a new one, or to prefer (on safety grounds at least) an airline like Ryanair over (say) Air France or Korean Airlines.
Whether it is irrational to refuse to fly on a new plane (or with a particular airline) is a harder call, because statistically the chance of dying on a flight are vanishingly small.
Anyone who has doubts about Boeings design ethos should look at the pictures of BA038 and the Asiana plane in San Francisco. That the fuselages of both stayed substantially intact is a huge tribute to the designers.