Can we conclude that a monopropellant system is necessary for ALSA to adequately reduce cost and complexity?
I don't think so. DARPA doesn't do many projects where we know enough to conclude a particular approach must be used; instead we don't know ANY approach that works, and particular teams take a shot at the problem with a high failure rate.
I think monoprop was just a choice by this team. That choice was probably driven by looking at how energetic this propellant was, then seeing it might work as a mono, and looking at the cost and complexity reductions that resulted. But now, looking at propellant stability, it might make more sense to separate the energetics.