There is no upper limit to something. The possibilities are endless …When there is almost no limit to how far you can go in what you are doing.
Example: “After I graduate from business school, the sky’s the limit!”
The adoption of the expression was no doubt influenced by the invention of the aeroplane. The phrase was picked up and used as the title of a Fred Astaire/Joan Leslie film at another time of intense interest in powered flight, in the middle of WWII – 1943.