actually that was crystal that took you back to Kanto and this is a gold/silver rermake so no. they dont have to put kanto back in.
Uh, did you PLAY Gold and Silver? You could go to Kanto there too. Crystal was only marginally changed from Gold and Silver in the addition to a female player character and more backstory about the Unown and the Beasts, and the addition of the first Battle Tower.
Anyways, there is nothing to say that the whole entirety of Johto/Kanto can't be redone in this generation. In fact, since it's now on the DS, they could do what Fire Red/Leaf Green did with the GBA and make this Kanto better(even include the Safari Zone!) because of the memory contents. They could make cuts elsewhere(cutting out contests and Pofin mini-game, like they did in Fire Red/Leaf Green) to conserve memory.
EDIT: Sorry, I had to put this in; if you remember, in Fire Red/Leaf Green, Pokemon were unable to evolve to new forms until you got the National Pokedex. They could apply that in the remake by making it impossible to evolve Pokemon to their new forms until you get the National Dex, which would only require seeing all the Johto Pokemon. This would prevent people from trying to evolve Sneasel or breed Sudowoodo.