Replayability comes from scenarios and differing objectives, at the moment, the games most people are playing with Age of Sigmar, indeed all reports I've seen about it anyway, are straight line up and fight, this is almost never the case with military engagements, even in a historical sense, the idea of two armies just lining up on a field being the primary way a war is fought is a fallacy. Its also pretty boring to play and watch.
Interesting, tactically diverse scenarios are coming this week in the new book, with the promise of more on the way from various sources, White Dwarf more Campaign books etc. and the game expects you to make your own with your opponent.
This is where the challenge for those who like tactical games will come in, pitting yourself against opponents with different goals or with asymmetrical forces, while still being narrative in that the aim is to tell a story.
If you're just standing in a line and moving forwards toward the fighting thats A) not using tactics, military or otherwise B) not particularly interesting.
8th Edition WHFB was dull because people only wanted to play that one mission over and over at a set points cost (2400 in my area) every game people just did the same thing to try and beat the opponent. I'm glad thats on its way out. The game wasn't narrative at all, it was my army lines up and fights your army and difficult to make it anything else.
I've attached two pages of a Citadel Journal article by Jervis Johnson, it's been shared about a few places but I've not seen it here, but here is an article by Jervis (now manager of long term strategy at GW) that points (no pun intended) to the type of game he prefers, reading this (written well over 10 years ago) it looks like that strategy is finally coming to the fore, anyway, read it, digest its wisdom and learn to accept that 8th edition was less of a true and real Warhammer Fantasy Battle game than Age of Sigmar is.