They were the leaders in a sector that didn't exist so in terms of marketing and business strategy we call them the "firsts" because they were the first to enter to be in this market. And if you want to know what McDonald's has done right to be so successful, the response is: do the things right satisfying the customers' needs in the best way possible.
One of the key features that McDonald's introduce to the market was the Happy Meal and the parks were the children play, that let the children have fun before and after the eat. So if they have fun, they will ask to their parents to go there, and as a cheap site to eat, the parents were and are happy to go there. But in the last days has appeared the problem of the obesity in a lot of countries and this make the people think two times before they go there.
In 30 years we don't know because the competence are so aggressive in the marketing way and the quality is similar so. But most important of all, is the customers' needs, and in that way if their needs are covered by other company better than McD then McD will lose this fight.