The possibility that anyone can easily replicate any startup scares A16Z.
If AI gets good enough to replace all human labor then actual physical moats to keep the hungry, rioting replaced humans away will be the most important moats.
If AI renders human labor a cheap commodity (say you can orchestrate a bunch of agents to develop + market a Windows competitor for $1000 of compute), what used to be "Satya + his army vs. you" now becomes mostly a 1:1 fair fight, which favors the startup.