Other than layering - flash NAND has already hit its limit for how many bits per cell we can deal with without it failing over night or getting any slow. Same with getting any smaller as the cell just keeps losing more and more life. If Xpoint can still scale from where it is now - even taking a bit of a hit in speed -it would still out last what flash NAND can do now.
I don't follow why Xpoint can't do more than one layer though - that seems like a false statement? Sounds more like they just haven't explore the idea yet than a real limitation. Heck they could even do layering over layering for both tech and stacking on top of that by building layer one half way - work on layer 2 half way - then finally layer 3 half of layer 2 etc etc till a "stack" and then stack the stacks layer down the line.