First, Garp was never, ever one step behind Roger. They were evenly matched their entire careers. Also, your speculation about is off. I said that Law is going to be, not currently is. By the end of the series, Law will be comparable to WB, that has nothing to do with where he is now. Zoro may be on that level, but he won't be any stronger than Law.
Then how come Garp could never beat/capture him?
And how do you compare Law to Whitebeard? Whitebeard was just as strong as Roger. Now obviously Roger is a modest man, and was probably slightly stronger than Whitebeard, but the legends tell a story of the two being the strongest, equal in strength.
Law is significantly weaker than Luffy. And as we know, Luffy will continue growing the fastest, to become the pirate king. So, with Law already behind, Luffy will keep getting stronger and stronger than Law. Luffy won't have someone who is equal to him in strength at the top.
Especially not with someone like Usopp on his crew to tell the tales to the world about how Luffy destroyed everyone in his way.















