I was thinking about this today. If the 'true' course of historical events was always that the Enterprise C vanished at Narendra III, as is implied by the episode (returning them home is what 'resets' the timeline), then presumably the war-torn universe is the true one. The more peaceful timeline, the one we think is the real one, the one we've been watching for three seasons up to that point... might just have been a peek of the universe as it should be. "Yesterday's Enterprise" allows us to pierce the bubble, and it shows us what truely happened, the true course of events, until the Enterprise C got sent backwards in time and made everything how we perceive it to be instead. It made my head spin a bit, but the more I thought about it the more sense it seemed to make. I think there's a logic there. The Enterprise C being sent back doesn't reset the timeline, it ensures the timeline is the happy one we've been watching all these years, instead of the crappy one that has actually been happening up to that point. Does this make sense to anyone else?