I can explain to you why i think it will be through Kid.
Lets take completely by side the motivation Kid has about Shanks. This is a big triggerpoint for that idea, but also from another POV.
But let me start from another point:
I think the final war will happen between Imu + Holy Knight + Gorosei vs Luffy + Crew + New Generation crews
So i expect Luffy will lead into the final war with Blackbeard, Kid and XXX (later in this theory) by his side, kinda as rivals, but not as enemies.
These 3 will come together on Laugh Tale, or on the island straight before and they will fight each other, for different reasons, but mainly the fight will be triggered the first time by Luffy.
Luffy will know about Blackbeard being the reason Ace was captured, he knows Blackbeard is the reason for whatever is happening to Garp and Coby and he will know Blackbeard is the reason for what happened to Law, maybe even several things more. Enough to get Luffy heated against Blackbeard, the first time for selfish reasons.
And here comes the point why i think Kid ultimately needs to beat Shanks. Because that will be the trigger point of Luffy being hella mad on Kid. Maybe Kid tries to explain, but mostly he will be fine with the fight. And so there begins a 3 sided fight, which will look impressive.
But the fight will be disturbed finally by someone else, someone convincing these 3 to work together. It will be Buggy.
Buggy will reach that point and explain to Luffy alot of things about the world, Shanks connection to the world government etc. pp.. Which will make Luffy mad and forces his will to talk to shanks, but it will lead all 4 of them to work together against the world government, Imu etc. pp.