What would happen if I ate nothing but mac and cheese for the rest of my life?

"A good source of protein, calcium and phosphorus, cheese can certainly be considered a nutritious food. That doesn't give you license to eat as much cheese as you want, however, because it also contains saturated fat and sodium, which aren't good for you. In fact, eating too much cheese can do more harm than good."

www.livestrong.com/article/504830-what-does-too-much-cheese-do-to-the-human-body/

You'd probably be fine for a while, but your digestion would get wacked up after a few weeks. You might miss out on some trace nutrients and develop odd issues eventually. I would not recommend this diet.