If we assume that free will doesn't exist (which I don't agree with, but for the sake of argument we'll assume it doesn't), one might still expect people to make choices that maximize their own well-being and it isn't too much of a stretch to suppose that such choices are more often than not good for society, or at least more so than a planned society where most choices are made for you by some authority.
One can argue for democracy, free speech, and other modern liberal ideas on the basis that they produce a more stable, prosperous society.
This assumes that we've already decided that stability and prosperity are attributes that society should have.
I think science is a great tool for figuring out what the consequences of a particular action are likely to be, but it's not a tool that will tell us which of several outcomes we should prefer.
Optimism and Hume notwithstanding the outcomes regarding prosperity and stability may be bestowed upon the society as a whole without the choices by individuals having individually assisted their own life outcomes.
China seems to be a counter example to that (and the largest example in the history of mankind, with 600 million people having a prosperous life thanks to, arguably, the government making decisions on their behalf, not to mention it has been the most stable civilization on Earth despite a few periods of instability), though on the whole, you might be right. I just wouldn't claim that with nearly as much confidence as you do.
One can argue for democracy, free speech, and other modern liberal ideas on the basis that they produce a more stable, prosperous society.
This assumes that we've already decided that stability and prosperity are attributes that society should have.
I think science is a great tool for figuring out what the consequences of a particular action are likely to be, but it's not a tool that will tell us which of several outcomes we should prefer.