I think if Big Mom falls, Katakuri and the big mom pirates are getting destroyed.
I don't know about that. Oda is very ambiguous about Katakuri: on one side he seems to be useless after WCI, on the other side Luffy's relationship with Katakuri are going to be important for the sake of the Big Mom pirates.
Katakuri was clearly worrying about Luffy based on his conversation with Brulee. He was holding back against Luffy and didn't really show his full arsenal of abilities–mostly copying Luffy's own techniques, showing how to improve his abilities further. (see above☝️) He essentially trained Luffy and even motivated him during the fight. Luffy and Katakuri have a primes to fulfill: Luffy promised to defeat Meme and Katakuri is waiting for him. So Kid defeating Meme is a big question for me to say the least, I don't see Law and Kid defeating Meme, and Oda's early-on set ups are showing it.
I'm not saying I want this to happen, but Oda is very repetitive in his writing, you kind of see where he can go–if he wants to–with Katakuri: and this foreshadowing with Katakuri becoming the next captain after Meme's death is a reinforcing sign to it.
Also Linlin had a lot of enemies and Kid is a CoC user. I think a lot of powerful people will flock to his crew after he beats big mom with Law.
I don't know who and when, but with every chapter In Wano, I fill like Oda is pushing the idea that Kid isn't ready yet. I can see Kid having some sort of timskip after Wano: recruiting new strong crew members, training his haki, finding out about CoC etc. And replacing Shanks after his death. Seems like a possibility to me!