I think that when people say "It's impossible for the powerless to become powerful", they really mean "It's very unlikely for me to become powerful", which is statistically true. Powerful people are rare - that's what it means to be powerful - and so no matter how much mobility there is at the top, the odds of any one person making it there are quite slim.
Then there's the controversial question of whether a monarchy's efficacy may in fact be greater than that of a republic, a topic that authors such as Erik von Kuehnelt-Leddihn and Hans-Hermann Hoppe have explored.
I think a lot of the confusion is because power operates as a hierarchy (or oftentimes, a graph), and each layer of the hierarchy tends to oppress the layer below it while itself being oppressed by the layer above. Hence you had poor blacks in Reconstruction being oppressed by the KKK (largely composed of poor white sharecroppers), who were being oppressed by the Northern bourgeoisie, who were being oppressed by Gilded Age industrialists who were being oppressed by populist politicians who were allied with the poor white sharecroppers. That's why you may have a monarchy with strong support from the common people: the monarchy keeps wealthy business owners in line, which keeps them from oppressing ordinary citizens. It also explains the current political situation in the U.S, where the Republican party is an uneasy coalition of wealthy plutocrats, small business owners, and poor rural religious whites, while the Democratic party is an uneasy coalition of academics, white Yuppies, unionized black factory workers, and immigrant Hispanics. The plutocrats oppress the yuppies, who offend the small business owners, who oppress the immigrants, who take jobs away from the rural poor, who discriminate against blacks.
It was an enormous risk for Thomas Jefferson, Ben Franklin & company, but the upside was huge. The average worker not so much. Non-landowners weren't even allowed to vote!
There are certainly examples of individual sharecroppers who attained power as a result of revolution, eg. William Few. [1]