People are to engage in Yonko/Admiral wars, but the fact that Oda went out of his way to show that is huge. You do wonder what exactly is the plan here.
there is only 1 possible plan , every major arc has a major plot twist planned WAY ahead of time like Ace death , i think Kaido and BM come back is just that , Oda went out of his way to show that their defeat had lots of ??????? and still followed the routine arc structure , this way he sets up expectations , and when everyone thinks they know where the arc is going , BAM you don't know what the fuck is going on
1) Oda makes sure that Kaido ultimate form that uses against Luffy is a MAGMA dragon and then throws Kaido in Magma
2) BM EXPLICITLY says "don't think this is enough to kill me"
3) Doesn't show BM and Kaido awakening FOR SOME UKNOWN REASON , the biggest villains in the series so far don't have an ability that Doflamingo had , of course it makes no sense , but he knows that readers that pay attention will know something is coming.
The Kaido and BM comeback is certain