That's why an Emergency Use Authorization is just that - there's an emergency. The FDA is well aware that long term safety is still unclear, but given the circumstances, the risks posed by vaccination for the general public are outweighed by the benefits.
" but given the circumstances, the risks posed by vaccination for the general public are outweighed by the benefits. "
That's the problem, we don't. We don't know what the risk is long term, it is unknown. It is illogical to say that an completely unknown risk has any logical comparison.
This is more uncertainty than the FDA would accept in the usual course of events - but this is an unusual time. The FDA has no adequate, approved, and available alternatives to the emergency authorization when faced with the pandemic. The investigational vaccine appears to be safe and effective against COVID-19 for the general population. Yes, the vaccine poses some unknown risks, but they are very likely vanishingly smaller than the very present health risks posed by the pandemic. For any given individual, they're much more likely to suffer negative health outcomes within the next 5 years from COVID than from the vaccine. Nobody is stating that risk to be zero. But that also doesn't mean "the risk is completely unknown so why even bother?"
How is the emergency authorization any different from an approval? If tomorrow an effective treatment for COVID-19 were to be approved or the pandemic ends, the EUA for the vaccine would be rescinded. Pfizer/Moderna must also track any adverse reactions more closely, and report them to the FDA on an ongoing basis. They also cannot sell them to private players - they're being provided to the FDA & their authorized parties. The vaccines are not licensed for COVID or any other use.