First off, how far will ND fall if what you mentions happens? Not very far at all. They would still have a top 15 class which is a good class.
Second, if Kelly jumps to the NFL, what coach do you think ND is going to have quickly lined up? They haven't quickly lined up a coach since the O'Learly debacle. Even if they chose a coach, their background checks would easily carry late into January.
The class may not fall apart since they've grown pretty close, but some of the top out of state targets could easily be picked off.
Michigan's class was garbage before they fired Hoke - they've completely lost the 2015 class by waiting this long to hire a coach.