Cohort studies provide suggestive information for generating hypotheses and designing randomized trials.
Cohort studies can only detect large differences.
But the kind of strategy treatment effects we're trying to look for are unlikely to be as large. You need to be sure that your 'signal' is greater than the factors that influence your choice of early vs. late. Cohort studies, unlike randomization, cannot control for unrecognized covariates. The integrity of randomization is only preserved if you follow everyone. My idea is to randomize and maximize follow-up.