I thought Megatextures were more to avoid the tiled look of many textured landscapes at the time. The idea that the artists can zoom into any point and paint what they need without worrying that it will then appear somewhere else on the map.
Looking around, some people seem to think they were replaced by virtual texturing, but I've been out the loop for a long time so haven't really kept up with what that is. I assume it allows much the same, but far more efficient than a giant texture map. Death Stranding is an example that must use something similar, because as you move about you wear down permanent paths over the landscape.