That. Moreover, aren't Warcry warbands incorporated into the "main" AoS chaos faction? Also, Underworlds gangs are in the battletomes so the only fantasy looking "not mainstream" game that has no models incorporated into AoS is Blood Bowl.
Maybe an unpopular opinion would be that there is, in the community, much nitpicking and lack of perspective; I mean, we are contemplating things in the now when we should be looking at them in the long run. In six years, we have seen nicely revamped or wholly new ranges (and then, those being expanded) like Stormcast, Kharadron, Fyreslayers, Soulblight, Daughters , Flesh Eaters, Bonesplitterz, Ironjawz, Ossiarch, Idoneth, Lumineth... Frankly, even the armies which did not get such resets are likely due to do so sooner or later. Then, the fluff is actually nice; IIRC the idea was that Skavens were able to "gnaw" their way across the Realms, and they are a main character of the biggest feature in the lore of late.
Just my 2 cents on that.