Do you even read manga ???
Saying that there are no competitors is laughable, literally every pirate is competitor to Whitebeard , fact that no one managed to surpass / reach his level is another matter.
1. ) Whitebeard is not just holding WSM title he is literally called world strongest pirate by Oda himself at the beggining page of Paramount war volumes. Oda didnt write this in a manner of title like WSM but as an fact . In manga Buggy said the same that WHitebeard is strongest pirate alive. Every pirate falls into this category so therefore they are all his competitors .
He is said to be only one capable of fighting equally with Roger.
Crocodile said that there are thousands and thousand pirates at sea who were not able to beat Whitebeard and Roger.
2. ) Entire era was called Whitebeard era in manga . Chapter 573 is titled "The Name of This Era is "Whitebeard" .
3 . ) He is man said to be powerful enough to destroy the world, its not title someone gave him it was stated as fact.
So WHitebeard being called world strongest man , or king of the sea are actually said as titles but those titles are well deserved.