Technically there is no difference between vertex data and texture in the context. Geomorhping regions require special handling. Rest is same. When blending 3 layers per triangle, of course, you do not have fine grained control of the blend and LOD morphs are visible. But generations of games were done, using this technique, even before texture arrays were introduced and it was fine. What is more important, that it actually runs, not crawls. But it requires changes to landscape and painting tool, which are not backwards compatible. I bet that Epic would not even consider going for that, and it is absolutely justified. Especially considering that virtual texturing has appeared on the roadmap.
However, you can still use existing system to encode data your own way in landscape weightmaps and have exactly the same system. Have 3 weightmaps, each defines ID of a slice(Probably can actually pack it tighter and use 1 weightmap for IDs of two layers) and have 3 weightmaps, each defines weight of the layer. If your texture array has 16 textures, you have 16 layers at your disposal at a fixed cost of 3 layers. Nothing stops you from fetching different arrays for each layer. At this point, neither POM, nor Tess or 3 way texturing or distance blending are that dreadful any more. Additionally, good luck trying to compile a more or less advanced landscape material with 24 layers. This is simply impossible. With arrays, it is just 1 shader permutation for all. Lastly, and most importantly, terrain cost is consistent. You do not get abrupt changes in frame time, just because a landscape component with 6 layers silently drifted into the view.
As for PIA with painting with this,indeed it is so, but exporting weightmaps from content creation software for this method is not hard, and getting used to painting like that takes few days. On top of that, changing the painting tool per project is trivial, unlike whole landscape code. Physical surfaces are also a problem, but ability to bake them from landscape material had been also request since first versions of UE4.
I will try blending layers based on frame, when I have some spare time, just for a sake of experiment, but I think the results will be disappointing.