I do believe that positivity will attract more positivity; however, positivity is not the same thing as success, or even happiness. I think of it more as being about hope: When you choose to see the good in an otherwise hard life, that's being positive, and in so doing you also acknowledge that more good is yet to come; you know this because there's already plenty right here, right now, if you look for it. Positivity is a candle that doesn't dispel the darkness, but gives you a little extra light by which to navigate it--and it's a promise that you can always light that candle whenever you want.
So to me, being positive isn't about attracting good things, and I don't think positivity is even capable of that. As you point out, plenty of positive people are suffering and plenty of negative people are living it up. However, it can make your world a little brighter, and hopefully enable you to help light other people's candles. In return, they light yours again, and it keeps going round. In that way, I do think that being positive will attract both a more positive (or accepting, even if not happy) outlook and more positive people.
I guess in terms of prosperity the idea of positivity comes down to self-worth. To use the same example the good person that is broke doesn't believe they deserve wealth. The bad person that is rich believes they deserve wealth. Again, I don't know that I buy a lot of these prosperity principles. I just don't think you can attribute low self-esteem to why things don't work out as one would like.