That greatly reduced the interest of second and third sons to seek glory or success at the Watch, now filled with common criminal scum or zealous religious fanatics, which also deterred smallfolk from volunteering to work in the Gift. And with less available food and land, the North could poorly afford to take care of the Night's Watch when they could barely make do by themselves. And now with the North spent after getting dragged in a southern war and subsequently invaded by ironborn, the Watch was on its last leg, they simply didn't know it yet. Maybe if Jon took back control of the North, he could divert some aid towards the Watch. He did decide to allay some of the worries of his friend.