> This is a cohort study conducted over 15 years. You can't really criticize it only based on those numbers
You absolutely can.
Scientific studies should be judged on their potential to produce interesting results. This study has too small a sample size to provide results that merit making even the weakest claims about. Even the initial sample size was, frankly, just barely large enough to maybe provide suggestions for future research.
If a meaningful drop-out rate is expected, I would argue that the researchers running these sorts of studies are mostly wasting everyone's time and money for the sake of publishing a few papers.
You absolutely can.
Scientific studies should be judged on their potential to produce interesting results. This study has too small a sample size to provide results that merit making even the weakest claims about. Even the initial sample size was, frankly, just barely large enough to maybe provide suggestions for future research.
If a meaningful drop-out rate is expected, I would argue that the researchers running these sorts of studies are mostly wasting everyone's time and money for the sake of publishing a few papers.