Seemed like Han, Luke, and Leia did alright for a decade or so. It wasn't until Ben turned to the Dark Side that things fell apart for them. The Rebels won the war. The Empire was defeated. The Republic was restored. Three decades pass, and the hardliners on the First Order have been rebuilding quietly and strike back at the Republic they hate so much for messing with their nice orderly society.
How much damage the First Order has done is not clear. They blew up the current Republic capital and four other planets in the same system as well as the main Republic fleet. The Resistance took out the superweapon but now have only a half dozen to dozen Starfighters left at their base. Six X-wings are not going to be enough against one Star Destroyer much less a fleet of them. So hopefully someone in the writing staff was smart enough to point out that the Republic has to have more than just what we saw, just like we would guess the First Order has more than what we saw. The First Order is suppose to be a relatively small rump state compared to the Republic and supposedly the remains of the Empire that still hold to the treaty.
Luke did set out to rebuild the Jedi order, but had a major setback. Han and Leia seem to have gotten married at some point, than separated over the lose of their son to the Dark Side. Each dealing with it in their own way. The EU did this too. Han and Leia going their separate ways over first the loss of Chewbacca and then over their son. They make up later to keep up the fight, but here, that is no longer an option. Luke had student turn on him in the EU. Some even tried to destroy all the rest. But because the EU was still going to be the story of the Heroes of Yavin, Luke, Han, and Leia needed to continue on. The EU did try to move on with the Solo and Skywalker children, but the Heroes of Yavin were still the main point in most cases. Now, with this film, it is no longer their story. The children are the heroes and villains. While the old Heroes of Yavin still have their place in the story, it isn't their story anymore.