I believe you just answered why every 20 or 30 year old should get vaccinated now. They're not going to be 20 or 30 forever.
Soon those 30 year olds will be 40 year olds. Then 50 year olds and on to 60 year olds. There's only 1 alternative to this progression and it's not great.
So, get the vaccine now, when it's available everywhere, completely free, and help to eliminate the spread. Plus, you get a cool sticker.
Or, don't. Let COVID linger and continue to percolate through the population. You'll get it eventually. Some new variant that's taken over by the time you get it. Bonus, if you manage to avoid it long enough you'll no longer be in the 20-30 year old range. Instead you'll be in a much riskier range.
Wait long enough, assuming you remember that you're ageing out of invincible, and it's not going to be free anymore. May come with a health insurance penalty. It's probably a guarantee that it'll come with a life insurance penalty. That life insurance may not matter to the 20 year old, but as they age and have kids it's going to become important.
So, get vaccinated. All the 20 year olds will not be young and invincible forever and you're going to want it before that happens. No sense waiting.
No altruism required here. Completely self interest reasons.