Companies aren't maximizing their financial benefit for producing the vaccine so that's one reason to not lift the waiver of liability. But to imply companies producing the vaccines wouldn't suffer enormous reputational/brand damage if their vaccine went south would be naive. The liability waiver only protects them financially, no way to protect them reputationally, and that is good incentive for them to get it right. Is it perfect? No but its not nothing.

Also you state the long term effects of the vaccine are unknown, but isn't the same true for the long-term effects of COVID itself? Since both are unknown I'd say it's a wash.

Oh and I'm neutral/undecided on the original argument for/against vaccinating kids.