I still don't see where you see that in TLJ/TFA, though. The good guys will win and nobody that really matters will die. The bad guys will immediately stop all action once their main baddy boss becomes good or dies, or does both in succesion. Such it was and so it shall be. "Good will triumph" isn't really some cerebral postmodern theory. Even the most snooty postmodern will have a degree of self awareness; to find that any of these movies are is, if true, likely a peak of irony never seen before.
You (probably?) missed the part where I talked about the writers.
Whatch this video (again if you havn't already), where David Stewart explains his views and how he came to that idea, if you so will. He doesn't call the movie post-modernist or nihilistic nor that it aims to be such, just that there are post modernist/nihilistic influences, like a kind of undertone, which might come from the writer(s) following maybe a nihilistic/post-modern approach, where they TRY to write a good vs. evil story with a narrative where the good guys always win, but actually fail with delivering a compeling story, as a nihilist would not grasp the concept of moral absolutes. For them the Sith and Jedi are simply the two sides of the same coin, and it doesn't really matter which philosophy you follow in the end as it's all pointless in the end anyway! - Remember the dialog in The Last Jedi? Something along the lines of "Today they shoot you, tomorrow you shoot them, all the same". Wow. Very Star Wars like dialog.
For example, what will it mean for Star Wars 10 if the 'good' guys win in Star Wars 9, and the Second Order of the Empire strike back in SW 10 with yet another superweapon, bringing the republic yet again to its knees, Leia now comanding the nu-nu-rebells from her wheelchair, so Rey's trans-gender-mary-sue apprentice fights the son of Fin and Rose who's now on the dark side (no phun intended), killing his father, while some unknown super evil master-mind, no one has ever seen before, works somewhere in the background to take over the galaxay ... you get the idea.
If good would have triumphed in this franchise, there would have been no Star Wars 7, if you think about it.
But that's not the point anyway, it's more that when you look at the first 3 movies, they are so simplistic in their moral narrative, where you don't even need to see the characters, to know from the first 5 min. of the movie that the rebells are the good underdogs and the empire some evil nazi-like entity just from the size differnce of the ships and how they look -
an extremly well done opening by the way. You will find no trace of moral relativism in either of the movies, the motivations of each character are always absolute. Vader as one of the main figures of the empire lies, kills and manipulates/intimidates where as the Jedi and the resistance with Luke and Leia are good, pure, disciplined etc.
The Last Jedi paints a very different picture particulary with Kylo, his conflict, Luke and his feelings for his nephew, Han and Leia braking up, and most partiuclarly the hacker and his role/dialog with the whole nonsensical sub plot. There is a very clear try of getting some 'gray' moralitiy into this, which simply put has absolutely no place in a Star Wars movie to be honest. You don't show Sauron buying his weapons from the same black smith like Gandalf just for giggles to show, yeah! Look Gandalf is not always super good.
Again, I am NOT(!) saying The Last Jedi is a post modern movie. Just that it has a lot more post modern influences compared to the first 3 movies of the franchise.