Plagueis/Sidious were responsible for the imbalance of the force. With them gone the power that manipulated them would be gone as well. Sith being gone is not the same as the Dark Side of the Force being gone, just the cause of the imbalance being gone - and probably also the balance restored.
You may also not misinterpret Dark Side Users as Sith. Sith are not all Dark Side Users and Padawan and Jedi are not all Light Side Users. Those are ideologies and not the terms for everyone using the force. Kylo Ren is also not a Sith (except if Snoke is one). I know in general those terms are used, I also used to do that but my brother who is more familiar with Star Wars told me.