Ken Griffin, the founder of Citadel, has cautioned investors about the future of the U.S. economy, predicting modest growth and highlighting the mounting national debt as a significant issue. What Happened: In a letter to investors on Monday, Griffin expressed his concerns about the economic outlook. He foresees a “modest” growth in the upcoming quarters, attributing this to the “challenging” economic landscape, reported Bloomberg. The letter, obtained by Bloomberg, also highlighted the U.S. national debt as a growing concern. Griffin pointed out that the net interest spending is projected to …