In actuality, I actually quit in frustration about 3/4ths through the game because I realized I could just blunder my way through the rest of the game.
Eventually I got tired of the game after I stopped caring about optimizing, and then wondered why I should bother completing the game other than for the sake of completing it. Yes there was a storyline to the game which should be completed, but the storyline was disconnected from the essense of the game (which to me was optimization). I would have felt more purpose in finishing the game if the optimization mattered somehow, like for instance if it affected the storyline or progress of your character somehow.
I don't think it is a matter to me of "external milestones" vs "personal milestones", but more about engagement and immersion in the story and world of the game, which I suppose is something I am after in addition to simple mental exercise.
For comparison, another Zachtronics game "SpaceChem" does a great job in linking the optimization into the game and storyline though the following simple mechanism: in order to defeat bosses, your factory had to meet certain timing requirements. To me, that requirement made all the difference in terms of immersion.