I mean, I would say that preventing 40% of disease (depending on what you're measuring, etc etc) is not marginal. I guess I'd turn it around and ask, if 40% is marginal, what number would not be marginal, and why do you draw the line there?

(For me, "marginal" would mean that the costs were roughly equal to the benefits. I think you'd have a hard time convincing me that saving the lives of even just 1% of the people who would normally die from the flu -- between 10k and 50k people a year -- is roughly equivalent to the cost of giving out flu shots. I suppose you could argue that the benefit is marginal to individuals who have a low probability of dying from the flu, and marginal to a society which has relatively low vaccination rates overall.)

The other thing is that, according to the Claude data here, the vaccine is actually relatively effective at preventing the transmission of some flu variants, 50% effective against influenza B! If so, there would be a clear group benefit to that, you only need (1 - 1/1.3) / 0.5 = 46% of people to be vaccinated to achieve herd immunity to that strain.