I am someone who has never taken an extreme position on this show. I never thought it was awful (in fact I found the early episodes fun) nor do I think it is any kind of masterpiece. For me the show... unfortunately and sadly... manages to be safely mediocre. The fault as I've said repeatedly lies in the execution because the ideas are good even if derivative. There's nothing new under the sun of course but it's possible to recycle old ideas if the storytelling is compelling enough. The show was never so terrible to turn me completely off and despite all my rants, I really enjoyed Cho Seung-woo's outing as Han Tae-sul... who never took much seriously and seemed to have a rip-roaring good time with the nuttiness of it all.
In light of the show's structural problems, I don't think the ending was terrible. What happened with Alice last year helped me recalibrate my thoughts on this one. It's fair to say that time travel (or the sci-fi genre as a whole) K dramas have an element of the supernatural to it. Whether it's God, fate, spiritual beings or some mysterious intelligence at play behind the scenes, that seems to be explanation behind everything despite all the initial scientific mumbo jumbo that's thrown out in the first and second acts. I have no personal objections to melding science with metaphysics as there's nothing inherently contradictory about that but it would be nice to give audiences something of a head's up on that.
To some degree Sisyphus does that. Sisyphus succeeds in ways where Alice fails. There are hints of supernatural forces at work before the big reset at the end. The church is symbolic of that. Sigma himself clues us in that there is something special about the church. That's why he built the uploader close by. The church where everything always ends in the time loop is the one surviving structure when war comes to Korea. Sigma touts the word "miracle" when he refers to the church. The final confrontation has always been at the church (the memories, the photographs of the leads at the altar point to that) in which Tae-sul is offered the zero-sum choice of "the girl or the world". A sacrifice has to be made at the church and at the heart of Christianity is the sacrifice of the Son of God, Jesus Christ who gave himself for his Bride... his church.
Although I don't think Han Tae-sul is any kind of son of God, his choice to sacrifice himself on "sacred" ground sees him resurrected with his bride to be. He's back on the plane given a second chance to live as a reward for finally twigging and giving himself up to save the "world". The power behind the universe has looked upon his choice and approved by giving Tae-sul's heart's desire. To live and be with the woman he loves.
To understand this, I take my cue from the old time loop classic Groundhog Day. Phil relives the same day over and over again until he learns the lesson that he needs to. He has to learn that he cannot manipulate circumstances to go his way with prior knowledge but instead to use foreknowledge for good. He also has to learn that authenticity and self-sacrifice is the key to relationship success. Putting others ahead of self-interest... to love neighbour as much as self. It is never said who makes that decision to set Phil on his course but there's little doubt that there's a moral law or intelligence that exists teaching straying individuals that life doesn't revolve around them and their wants. As a result of coming to that realisation, Phil gets the girl. To find Ms Right, he had to be Mr Right first.
As for Sigma, he represents a kind of restless evil that good must constantly overcome. The roots of which run deep. It is true that time travel has been stopped so that instrument can't be used but evil will find new ways to steal, kill and destroy. It's the old Manichean dualism of good and evil in constant struggle over time.