Recently I have had numerous reunions with Uncles from my extended family. Some of them are successful businessmen for decades. They built their business from nothing and are living quite a comfortable life now. We had long conversations about life in general, including their key to success. I knew very well that they have been hard workers ever since I had a memory so I would contribute their success to endless efforts. However, one thing they had mentioned in common was luck. They believe pure luck made them meet and grasp those opportunities and become successful rather than their smartness and efforts. I've thought about this before and couldn't agree with this more. They've set very good examples to me, including my father, and now I tend to focus more on work itself consistently and let luck or fate takes the rest.
I think that luck is the thing which creates billionaires. As for people who achieved financial stability and some grade of prosperity, this is more about dedication and being smart (as you correctly mentioned). Making efforts is, I think, enough to have a good, or even really high income, luck is the main factor in billionaire-scale earnings.
Billionares are created by hardworking people who are genious and who make use of every opportunity given to them, who have good leadership qualities who know how to make use people potential.
Luck can be a factor in success but that's secondary , primary thing is your efforts and determination.