The scientific explanation was not "obviously the Doctor grasping at straws." Star Trek
is a secular universe where everything has a scientific explanation; moreover, it's a humanist
universe, meaning that there's nothing ultimately beyond the comprehension of the human/sentient mind, just stuff we haven't figured out yet. It would've been going too far if Janeway had actually been converted to a supernatural belief. What's significant about "Sacred Ground" is that it got Janeway to consider the possibility. Her reaction at the end isn't really about the phenomenon itself or one explanation or the other for it; her reaction is about herself
, about how the events of the episode affected her way of thinking. Much like Sisko vis-a-vis the Prophets, she hasn't rejected scientific explanations, but has opened her mind to the possibility that the difference between a scientific explanation and a spiritual one may be more a matter of interpretation than of right vs. wrong. For a little while, she was able to think like a religious person, to take a leap of faith, and it gave her more understanding and respect for that way of thinking.