The United States will eventually adopt the metric system because the US is currently the only developed country to still use the Imperial system. The imperial system is outdated and inefficient, and the base 10 structure of the metric system just makes more sense. It would take a lot of planning for the US to convert, but it would save time and money in the long run.
The United States is one of the most stable countries in the world, mainly thanks to its wealth meaning it has a high degree of control over its destiny. With such stability there is no need to change things such as metric systems. As we stand today it works fine, therefore there is no need for the U.S. to adopt the metric system.
After operating with our current system for more than two hundred years, I think the US is very unlikely to ever switch to the metric system. For one reason, I think the US prefers to be a trend setter and not a follower. I also think the sheer expense of transitioning the entire country to the metric system would not be well received by tax payers.
The cost of adopting the metric system for the United States is prohibitive, when you consider all the conversions for things such as gasoline pump systems and package labeling and instruments used for weight and measure. It would be too expensive and disruptive on our economy and Americans would be adverse to learning the new system.