Because they trained everyday and Gohan slacked off, if Gohan trains like his father does then he'll be the strongest. Similarly, Goku and Vegeta will eventually surpass Beerus since they are training under Whis who made Beerus a G.o.D level.
The question was if Gohan was stated to have the most potential out of all the Z Senshi, wouldn't he be stronger than Vegeta and Goku considering Elder Kai's potential unlock brought him to his limit, meaning he became as strong as his body allowed him?
He has a the most potential and will be the strongest if he maintains or trains it but he doesn't that's why he got surpass. Limits doesn't mean anything in DB.
Honestly I hope that he is a major player again, but I think all his potential went into Pan. Dude she flew out into space. Even baby Broly couldnt do that.