<<I can give you several examples of people who have been succesful while coming from meager backgrounds.>>
Of COURSE there are "examples" of people who have been successful who come from "meager" backgrounds. (One should add: there are also "examples" of people who come from affluent backgrounds and who don't make it, whose lives get destroyed in one way or another.) Especially, I think, for the generation who grew up during the depression and WWII. So many people had nothing. And when WWII ended, the US was the "last man standing" in the developed world. There was enormous opportunity, as we "rebuilt" both ourselves and many other countries as well. Plus the GI bill enabled millions of people who would not have had otherwise had the opportunity to get an advanced education. This was revolutionary. An example of government spending to help people help themselves at its best.
But it is also true that, of course, most don't overcome their "meagre" background. And most who begin with an affluent background do "succeed" in terms of employment, attaining some measure of wealth, etc. A few people say it is the "bell curve" and hereditary. But if you just ask yourself, if two kids of parents from opposite ends of the social spectrum are switched in the hospital, would they have had the same life, become the same person?