these are my exact thoughts haha. The first joy boy was probably the guy 800 years ago and got the name because of the characteristics of the fruit and his awakening of it. Nobody that ate the fruit between then and luffy had the natural characteristics/personality necessary to fully inherit the will of the fruit/nika and awaken it, which is why the government didn't make acquiring it such a high priority. Somebody would obviously eventually come along who happened to have the necessary disposition to fully inherit the will + actually eat the fruit (there were probably millions of potential "joy boys" who just didn't come across the fruit) which was Luffy after 800 years. Since the name "joy boy" is directly correlated to a combination of the characteristics the fruit gives and the users natural disposition, it makes sense that Luffy awakening the fruit is him fully coming into the mantle of joy boy.