I don't know if I'd say they made unrealistic assumptions. Geocentrism failed to make accurate predictions and required a whole slew of tricks to reconcile the problems. Heliocentrism was "unrealistic" because the church disagreed with it, not because it disagreed with observations.

It also took approximately 300 years for common opinion to shift from a Ptolemic universe to Heliocentrism. We're on a much shorter time scale for the Big Bang theory, and it's very well acknowledged the theory has problems.