Blackbeard didn’t make a sentiment about fire vs magma.
Akainu has a grudge against dragon who raised Sabo like a father.
Sabo vs Akainu is inevitable.
First you talk about whole brother thing now you are talking about magma vs fire which it don’t have narrative wise. By your logic, zoro will fight akainu cause he cut king magma.
If Akainu have mulit universe feats , you will argue Sabo will fight Akainu instead son Goku or superman.
Akainu literally have been targeting luffy for most of time . Whitebeard pirates knew Akainu want to kill luffy .
Sano raised by dragon for his revolutionary business , nothing do with family. We know Revolution
Agenda, so Sabo will fight gorsei or IM.
Saying Sabo vs Akainu is inevitable is like saying sanji vs king inevitable.