Here’s my opinion:
JB is close to ready to retire.
NCAA comes down hard on SU, to the point where it is obvious to everyone - most especially the AD and JB - that there a going to be a couple of bumpy seasons ahead.
AD and JB both know this would be a rough stretch to throw Hop into. It would look like a step backwards. AD knows JB can weather it, he has equity in the collective fan base. Hop would be judged more harshly.
Despite promises to Hop, AD asks JB to stay on and views this as a “re-stabilizing” period. JB agrees, but doesn’t really want to set a timeline on it. AD suggests 5 years so it cannot be used as a recruiting tool against SU in the short term. JB agrees.
Hop gets offer. Recognizes an opportunity. Both sides agree that the timing is good for Hop to strike out.
JB has to Coach a couple of years with limited scholarships, compounded by early departures yet still puts together a tournament team with string and duct tape. Established another top recruiting class to help ease the situation.
Bonus: JB gets to coach his own son for a couple/few years.
Bonus II - Hop takes over a P5 team in a conference that is in his home region and is ripe for the picking.
Just my opinion...