4) The Republic Is No More
In a direct echo of the original trilogy, The First Order use their new super weapon to end the Republic, destroying entire planets just to send a message to those who choose to help the Resistance. Now, we don’t condone the mass genocide of billions, but from an audience perspective, the First Order did everyone a huge favour here.
One of the (many) pitfalls of the Star Wars prequels was the time that they wasted on the convoluted politics of the Republic. These scenes added little to the plot and at the end of the day, they were just boring as hell. Now that the Republic is basically no more (we presume), more time can be devoted to the central narrative, streamlining our understanding of the Star Wars universe.
Whether any members of the Republic survived or not, the Resistance will now be more determined than ever to defeat the First Order, demanding justice for the deaths of millions. Inadvertently, the First Order may have also helped the Resistance in the long run, as parties who were previously neutral may also be driven now to join the good fight. In the meantime though, the loss of the Republic will undoubtedly be a huge blow for the Resistance, temporarily weakening their stance in the Star Wars universe.