Raw numbers collected over decades of research disprove this handily. While working hard is a factor, there are other phenomena which will just as easily impair upwards social mobility. There's a reason they call it "poverty trap"
For an introduction, take a gander at Gladwell's books.
http://monkeysuncle.stanford.edu/?p=541
Pinker’s term: “The Igon Value Problem” is a clever play
on the Eigenvalue Problem in mathematics. You see,
Gladwell apparently quotes someone referring to an “igon
value.” This is clearly a concept he never dealt with
himself even though it is a ubiquitous tool in the
statistics and decision science about which Gladwell is
frequently so critical. According to Pinker, the Igon
Value Problem occurs “when a writer’s education on a
topic consists in interviewing an expert,” leading him or
her to offering “generalizations that are banal, obtuse
or flat wrong.” In other words, the Igon Value Problem
is one of dilettantism.
As for the Horatio Alger "myth", the modern incarnation is the internet startup. Look at YC. There is just no dispute that smart people working hard can put a few million in the bank after several years of all out toil in the Valley.This statement is either utterly nonsensical and backed up by nothing or it's circular in nature (i.e. "How do you know they're hard working and smart? Because they put a few million in the bank", "Why were they able to put a few million in the bank? Because they're hard working and smart").
I suppose it depends on what you mean by success. But by the most common definitions (e.g. being rich) hard work isn't the main factor or even required at all (e.g. inherit millions from your parents requires how much effort? Basically not dying).
I watched my dad hold down two jobs and come home and work into the night providing his family with the things we needed. The first career job I had made more than he's ever made in his life and that job was mostly standing around.
Sadly, how we really wish things were has nothing to do with how they actually are.
I can "work" very hard pushing stones uphill like Sisyphus, and I probably won't get rich as a result.
A major reason why inequality exists is because the work ethic required for long-term thinking and planning is a model of behavior which children acquire from their parents through learning, observation, and imitation. Children from families where parents do not consider long-term goals worthy of pursuit are hugely disadvantaged.