It depends on who else would've/could've taken his place. Believe it or not, Tom Cruise was one of those originally considered for the part of Jack Dawson.
As for whether or not it would've bombed, who knows? I can assure you that the rates would've been different, but that's all I can say. IMHO, I think that about 60% of Titanic's success was due to Leo, but, out of the remaining 40%, 35% was due to Titanic curiosity and special effects.