That’s a good question. From what we can tell from his comments, the main drive of the five-year gap was to skip the bildungsroman section of Arya’s, Bran’s, and Sansa’s stories specifically. 

In this fashion, GRRM could use flashbacks to do a training montage that explains how Arya became a badass assassin, Bran became a greenseer/shaman/wizard ‘arry, and Sansa became a Machiavellian politician, without having to spend a lot of time describing the incremental process of maturation in detail.

It would also have changed their ages. Arya would re-enter the narrative as a 16-year old, Bran as a 15-year old, and Sansa as a 19-year old. This obviously would change what kind of stories he could tell about them: 15-16 is still a pretty young adolescent, but within the world of ASOIAF Robb Stark was King in the North at their age, so the reader would be more accepting of them changing the course of world events. Likewise, as adolescents, anything having to do with sexuality would feel very very different than it would without the gap. 

The problem, as GRRM found out, is that the gap doesn’t work as well for everyone else who isn’t those three characters, sending them on Odysseus-like extended sojourns where they would inescapably experience extended character stagnation. Tyrion’s drunken depression works in ADWD because (for him) it’s right after he’s killed his father and his lover – it would wear much much more thin if it had been five years. Jon and Daenerys’ ultimate failure as leaders make sense if they’re adolescents who have no experience in government and came to power way too early, it doesn’t work for people in their 20s who’ve been ruling their respective kingdoms for five years. 

So overall, I think he made the right call. If I were to have advised GRRM, I would have said it’s a lot easier to go back to your earlier books and do a find and replace on the ages of your younger characters, adjust your timeline a bit, so do that instead.