That is a nice simplification and does not really cover the flu and flu shots. That is why you can get 10-20 % effectiveness, because with the flu the system is a lot more complex and because of international travel and chance there can be several strains in one year and it is still a guess which one will be most dominant.
That is also why unlike with smallpox even the WHO does recommend the flu shot to groups with higher risks because the herd immunity with the flu is almost not achievable unlike with smallpox or measles. That is why usually with the flu the risk groups get the shot not everybody because it is very unreliable.
Everyone has the shot available - the vaccine is prioritized for risk groups, not set aside for them.
It is true that you aren't going to get complete immunity with a flu shot, for yourself or others. What you will get is resistance. That is worth it a thousand times over. If you're fighting a war, do you go out wearing a bulletproof jacket or not? I'd take the jacket.