It is my understanding that simply waiting for a longer time would not have helped. This side effect did not come much later after the vaccination, but only in very rare cases, which is why it took some time to notice it. Considering how many people we vaccinated with the Covid vaccines, we should have seen (and have seen, with myocarditis, trombosis) such rare side effects. Plus, it is likely that the disease itself would have a similar effect on patients, because it is an interaction of the immune system with the nervous system. The vaccine itself is cleared from the body within weeks, after all, so it's not like medicine that is taken over a long period of time and can build up.

So what you need is not longer trials, but trials with more people. Which is what is done in so called phase 4 trials after approval.