I think the biggest issue with taking out The Republic is that in TFA the Hosnian System is treated as The Entire Republic And Their Fleet. When it’s destroyed the audience is just left saying ‘Okay, I guess the First Order rules the galaxy now because the entire New Republic was just that one system’, even though in any galaxy that doesn’t really make sense.
Take Earth for example. We don’t have a one world government, but imagine if the United Nations was a bit more powerful, with a rotating seat of government which is currently in Switzerland. If a country like North Korea took out the UN headquarters and even all of Switzerland, this would be a devastating attack and throw the world into chaos, but I also wouldn’t expect the nations of the world to suddenly give in to North Korea.
This is essentially what has happened in TFA in the lore. The Hosnian system was just one of many potential seats of New Republic power, and the one currently guarded by their largest standing fleet. With this destroyed the New Republic is in chaos, but is not yet conquered. TLJ is actually as much to blame as TFA in regards to unexplained worldbuilding since it establishes that the First Order now de-facto reigns over the galaxy and they will take over all systems within weeks.
It would be like North Korea suddenly unleashing a hidden fleet of aircraft carriers across the Pacific to try and subdue the nations of the world.
Then TROS comes in with the Sith fleet idea and strains this analogy further.
Russia could have been building a fleet of 10,000 Soviet submarines in secret, each with a nuclear payload, and if North Korea allies with them after the loss of their aircraft carriers they can still rule the world.
But none of this really matters because each nation still has their own military, and once the world got its act together they would combine forces to take out this Russian/North Korean alliance which is only really a threat due to these outdated nuclear submarines that need external radar to help them navigate out of Russia’s only warm water port.
Of course, if you give the ST this sort of context it seems rather like the New Republic was always going to come back and win, which is why the biggest problem for me is not what happens but how it is portrayed. If in TFA they described the Hosnian attack as decapitating the New Republic government instead of destroying the New Republic itself, it would go a long way. In TLJ if they’d had just changed the exposition at the end from ‘allies in the outer rim’ to ‘central worlds of the New Repubic’, it would have shown how these otherwise powerful worlds were frightened into inaction by the First Order. Finally, if TROS would have dropped the planet-destroying capabilities of the ships while keeping the civilian fleet arriving it would have actually made sense.