I would submit its less "opt in only" and more because everybody has had the flu in their life.

I've gotten flu twice in the last five years despite having gotten vaccinated religiously every year; CDC is reporting this year's vaccine is only 45-50% effective. That said, even if you get influenza having the vaccine, symptoms tend to be more mild and resolve faster due to antibodies.

I agree that a lack of vaccine and clinically-effective antiviral treatment for SARS-CoV-2 is contributing to much of the widespread unease-nee-panic in the general public, especially in vulnerable populations.