Running a Galactic Government without Jedi to scare detractors seems to be difficult. The old republic fell to pieces when the Sith offered to get rid of the Jedi. The Empire needed to iron fist everyone into staying in line. Presumably the New Republic and the remains of the Empire were too evenly matched at the end of the Civil War for either side to win or hold dominance. They both consolidated their positions and became superpowers of a divided galaxy. Both sides use ships based on older designs but clearly updated and the resistance aren’t meant to be the main fleet of the Republic. They are an insurgence within First Order territory funded by the Republic with rather implausible deniablity