More like the other way around. The Sith are Dark Side practitioners, while the Dark Side of the Force might be called a force of nature, with little concern for the mortals which worship or utilise it. As far as the plot of the Saga is concerned, yes, it is the Sith who seem to be the only ones bothering to harness the Dark Side in any meaningful way, but once they're gone, the Dark Side will still endure, and the absence of one cult might as well be the same to it as the loss of a fingernail. I'm sure the ST will include such evil characters, but let's hope the filmmakers don't take the cheap option of labelling the next Dark Side users as 'Sith'. Similarly, the good/light side of the Force itself isn't necessarily bound to the Jedi, although the Jedi are bound to it. The abandoned concept of the Whills (yes, I know it can't be accepted as canon, but it's a reflection of GL's thinking) suggests that there are beings and those that serve them with a different - and perhaps superior - approach to understanding the nature of the Force. The PT did emphasise the fallibility and complacency of the Jedi, while in hindsight the OT does much the same to the Sith.