...Part of the problem with time travel throughout all of the series is if it is as simple as it appears why wouldn't they use it more frequently to save billions of people?
Generally speaking as a plot device I think Time Travel scenarios suck because of all of the holes it creates in the narrative.
Yes. It has always been a Star Trek thing to go back in time only
when it is required to move the plot along in the direction the writers want the plot to move along.
After some awful event unfolded in a Star Trek TV show where lives are lost, I would often think to myself (only half-jokingly) "Why don't they simply go back in time and fix that so it doesn't happen, and save those people"?