You bring up some interesting issues that do make sense, and perhaps they weren't after balance in terms of making the light side stronger. What if they wanted an equlibrium where there would no longer be a flunctuation of dark and light, but rather a mute point where both sides are at a stand still or nonexistant. In the end, that doesn't happen though as Luke is left and he is, by the movies, good. So I'm not quite sure what was meant there, but I do think their original view of things could have perhaps been to stablize the sides of the force.
That within itself doesn't seem like a horrible thing does it? Perhaps because of this threat they knew was coming, they knew they would continue to dwindle. Yes they were at a high point, but where do you go from there? There's no way they could continue to increase in power. Palpatine's power had already became a major force even before TPM.
Like someone mentioned earlier, that you did not seem to comment on. Perhaps they thought such stabilization would be achieved by this Chosen One, but by means other than evil. Regardless, we can't deny the prophecy and he did fulfill it. Just not in the manner in which they would have seen.
