Try to understand, they don't trust the government, they don't trust the CDC, they don't trust Pfizer and they don't trust you. Yes you have an official list of ingredients that someone with an appropriate degree can understand. But what's the stuff in the actual needle that you are so intent on making sure gets injected into them? Seems perfectly reasonable to them that some PhDs in some pharma companies are really worried about global warming and a coming Idiocracy. Why not put something in the vaccine that reduces fertility for the unwashed masses?
Maybe it sounds crazy to you and me, but a lot of people hear experts saying seemingly ridiculous things (like needing a vaccine for a disease you already got and recovered from), see how everyone falls in line with the narratives as they come and go, watch mainstream news and social media restrict all alternative points of view. And they don't trust the system. And its clear to them that this system they don't trust, also doesn't like them. From that perspective, of course they don't want the people who lie to them and hate them to have the authority to stick some chemicals and DNA (I know, mRNA) in their arm.
So who're left? The people who are concerned about vaccines because other people are, rather than because of (misplaced in this instance, but not in general) scepticism of the claims of untrustworthy, powerful people. Those people can be reassured by thought experiments, because they're assuming dangers due to incompetence moreso than malice.
¹: I suppose an argument from game theory might, but only if they think the vaccine-poisoning enemy is in it for the long run, so the threat of the truth coming out in (e.g.) 20 years would be enough to dissuade them from something like that. If they think the enemy's plans are shorter-term, or they can afford the loss of face, I don't think there's any way to convince them short of convincing them that the industry behind “non-addictive” heroin, thalidomide (developed by a Nazi war criminal!!) and the contaminated blood scandal is totally cool and in no way out to get you.
Worst-case scenarios should be constrained by your knowledge. I know what's in the Pfizer vaccine, so my worst-case scenarios have to be constrained by that. But sure:
“Worst-case”, somebody's poisoning the vaccines. This leads to severe side-effects from some batches, which the statisticians take entire weeks to notice because they're all asleep on the job. Vaccine roll-out stops pretty-much everywhere, but nobody's able to trace down the saboteur. A dozen new, highly-transmissible and virulent variants emerge, with different antigens to each other and to the vaccines, so vaccinating the populace becomes intractable.
You see that this scenario is incompatible with “everyone gets vaccinated”. The bad stuff largely comes from “people don't get vaccinated”; the only way everyone*'s going to get vaccinated is if the vaccines don't suddenly get poisoned or otherwise become dangerous.
* except the immunocompromised, and others who don't get vaccines for health reasons.
I can't do a “worst-case scenario” where the vaccines are already poisoned, because they're not. It'd be bloody obvious if they were, given how big a sample size the statisticians have to work with. That would be even more blatantly fictional than my “tens of millions die every year” scenario.