See, you're looking at it through a really cynical lens, like there's no point in telling Sage anything she deserves to know because "oh they'll probably break up after she graduates".
Like this, you make it sounds like MC should just fuck around while Sage is at work because there's no chance he'd get caught with her away, or he should dump Sage so he won't be starved of pussy for long stretches of time. Long distance relationships are hard, I won't deny that. But A) it might not be long distance, B) MC already knows what it was like and try to avoid the same pitfalls, C) MC and Sage are on the same page, unlike MC and Zoey a year ago. They want this to work, a family can be in the cards for them, but weighing the benefits of telling the truth about an ex wanting to get back together versus expecting the relationship to fail from the start like some business deal is not the way to go about it.