I got home late last night from seeing the movie and will echo similar sentiments in that the movie was good, not great. It had its moments, but left a lot to be desired.
With that being said, Murfdogg21's question/s actually has an interesting answer if you're a fan of the Star Wars EU.
While the first few points are rather fundamental and may even seem rhetorical, they are important to lay out before answering your questions.
Background:
The Force is the underlying connection between all living things, constantly seeking to find a balance.
There are individuals born 'Force sensitive' in that they have a better innate understanding of The Force. These individuals have the capabilities to manipulate The Force at their discretion.
Ultimately, The Force runs on a spectrum, but we often seek to simplify this spectrum as good versus evil, light versus dark, and etc. In reality, There is a lot more grey area.
The Jedi and Sith Order are organizations with a specific set of values and beliefs with regards to The Force. This is why they are often referred to as 'ancient religions' in the Star Wars EU, by non-Force believing individuals.
A Jedi or Sith is just an individual that receives specialized training to enhance their capabilities. However, these individuals can transition throughout their life as they move across this grey spectrum. A good example in the movies is Ben Solo the Jedi transitioning to Kylo Ren the Sith and finally back to Ben Solo the Jedi.
The Rule of Two:
In the Star Wars EU, the Sith Order was much more similar to the Jedi Order from Episodes I, II and III. There were many, many Sith. However, a key difference was the large amount of internal conflict among the Sith, as each Sith sought power, knowledge, pride, and etc.
A Jedi turned Sith named Darth Revan observed that the Jedi were organized as a group with a focused purpose and thus would always be stronger than the Sith while the internal fighting persisted. Philosophically he decided the best way for the Sith to gain power was to eliminate other Siths and concentrate their power in less individuals. Remember that The Force seeks balance and therefore if there are 100 Jedi with a power of 1 and 100 Sith with a power of 1, but 90 Sith die, the remaining Sith have increased capabilities with a power of 10 - or at least in theory. He felt this would help prevent internal conflict as would make it easier to unite the Sith Order into a common goal of dominance. Darth Revan never actually put this into practice, but left his philosophy behind at the Sith Temple.
Fast forward 1,000 years and nothing changed. The internal conflict within the Sith Order had grown as a whole the Order was weakening from the external (Jedi) and internal (Sith) conflict. Darth Bane studies Darth Revan's teachings and reaches similar conclusions, that the Sith Order will slowly destroy itself. He refines Darth Revan's beliefs and initiates a Sith purge, establishing a "Rule of 2," whereby there would only be a Sith master and apprentice at any given time, significantly magnifying each's power. After the purge, Darth Bane takes on an apprentice, Darth Zannah and thus the Rule of 2 begins in full.
From here on out, you more or less know the rest. Darth Plagueis was mater to Darth Sidius, and Darth Sidius to Darth Vader. With Vader's death, Sidius needed a new apprentice - which was supposed to be Rey. If Rey struck down Sidius in an act of revenge or hatred, she would inherit the concentrated power of the dark side. Hence, her vision was of herself and Kylo Ren joining forces to lead the Sith Order, which was Sidius' end goal. But she didn't, she choose the light. At the end when she kills him, it's as a Jedi and as an act of defense of the people fighting for their freedom.
I thought that part of the movie was actually great. Although I understand 99% of people don't know what's going on because this is knowledge you would need from the Star Wars EU and therefore if you were just watching the movies, you wouldn't understand what the hell was happening.
All of this also means that the Sith aren't gone, they're just gone for now. The Force seeks balance and therefore there will be those born that will seek out the dark side and will look to re-establish the Sith Order. Hence, the story is never done and there's an infinite universe to make movies on. The Star Wars EU is already extremely detailed and thus those were be interesting to see in the theater.