Yeah, and George Smith made the "unrealistic assumption" that the moon was made of cheese, and John Doe made the "unrealistic assumption" that dogs communicate telepathically...

While there are certainly a handful of "unrealistic assumptions" that have lead to scientific breakthroughs, giving credence to an idea simply because it is unrealistic is... well... unrealistic.

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.

Galileo's problem wasn't that he made an "unrealistic assumption", it was that he couldn't explain the lack of visible parallax. (It turns out the answer to that was that we didn't have good enough equipment.)