This reminded me of an old joke:
Two economists are walking down the street with their friend when they come across a fresh, streaming pile of dog shit. The first economist jokingly tells the other "I'll give you a million dollars if you eat that pile of dog shit". To his surprise, the second economist grabs it off the ground and eats it without hesitation. A deal is a deal so the first economist hands over a million dollars.
A few minutes later they come across a second pile of shit. The second economist, wanting to give his peer a taste of his own medicine, says he'll give the first economist a million dollars if he eats it. The first economist agrees and does so, winning him a million dollars.
Their friend, rather confused, asks what the point of all this was, the first economist gave the second economist a million dollars, and then the second economist gave it right back. All they've accomplished is to eat two piles of shit.
The two economists look rather taken aback. "Well sure," they say, "but we've grown the economy by two million dollars!"
And often if you box yourself into an API before you start implementing, it comes out worse.
I always learn a lot about the problem space once I start coding, and use that knowledge to refine the API of my system as I work.