Chapter 999 is one of the most important Luffy chapters in the entire manga. If you read closely, every scene is directly or indirectly proclaiming Luffy as Joy Boy.
Especially Whitebeard’s decision not to pursue Kaido.
Here’s how I see the sequence of events:
1) Oden and Roger discover the Will of D. and One Piece and realize they’re 20 years too early to be Joy Boy’s inheritors;
2) Roger declares his son will be the man Joy Boy is waiting for;
3) Oden refuses Roger’s assistance, vows to prepare Wano and “wait” 20 years for Joy Boy;
4) Roger meets Whitebeard and explains the Will of D, Joy Boy and One Piece.ù