Finally, the biggest risk of long term danger in any treatment is from long term use of the treatment. It's the treatment causing a tiny, undetectable amount of damage each time it's administered. So over thousands of uses it all adds up to severe issues. Let's take your nightmare scenario and say the vaccine became something that's administered every 6 months for life. You're talking about on average 150-160 doses administered in a lifetime. That's not that many. If the vaccine caused a small, imperceptible amount of damage each time it was administered, then 150 shots likely wouldn't add up to much. If it's causing enough damage that 150 doses would be a serious risk, that's something that would be caught as a known side effect.
We know way more about this vaccine than you think we do.
The vaccines will lose effectiveness the more they are used.