Since you think Kid and Law are Luffy's biggest rival and think Blackbeard should job to Law this late, let me educate you.
DRESSROSA ARC
This is Law versus Doflamingo.
This is Luffy versus Doflamingo.
WHOLE CAKE ISLAND ARC
This is Brulee gossiping about some Supernovas
including Kid versus a Sweet General.
Now here's her gossiping about Luffy versus the STRONGEST Sweet General.
WANO ARC
Kid and Law beating Big Mom
together.
Luffy beating Kaido
alone.
PRESKIP BLACKBEARD
Here's preskip Blackbeard doing what WCI Luffy could barely pull off.
POSTSKIP BLACKBEARD
This is Blackbeard right after the timeskip.
You notice a difference between Blackbeard and Kid and Law? Kid and Law have been consistently used as
hypetools for the next big bad and achieved
lesser versions of Luffy's accomplishments. Blackbeard on the other hand has been way ahead of Luffy since their starts as pirate captains (don't get me started on his crew being portrayed as "better" versions of their Straw Hat counterparts). Luffy doesn't have to catch up to Kid and Law. He has to catch up to Teach. If Luffy still has yet to catch up with Teach, why would Teach job to a lesser pirate than Luffy all of the sudden?