> If they were each creating the material world by observing it, surely they wouldn't be in such concord and we may not expect it to persist over time.
Why make that assumption?
As a card-carrying idealist, I think language such as “creating the material world by observing it” is best avoided. Here’s what I would say instead:
Minds and their conscious experiences are the most fundamental reality, to which everything else can be reduced and/or from which everything else emerges. Physical events, entities, processes, laws, etc, are simply particular types of patterns in the experiences of minds. The experiences of different minds are (partially) correlated-that correlation is itself another such a pattern. What causes that correlation? Well, causation is also just a pattern; causation is a type of explanation, but every search for explanations must eventually end in something accepted without explanation (the Münchhausen trilemma), so something things just are the way they are, some patterns just exist, without having any other pattern as their cause. It’s patterns all the way down, until we reach minds and their experiences at the bottom. That’s all we can ever know, and all there is to know.
From that viewpoint, talk about “minds creating physical reality” is a rather imperfect metaphor, true in some ways, but also prone to leading to dubious assumptions (like the ones your quoted argument relies on)
And if this is a “faith-based” position-well, no more than materialism is-it is just spinning the arrow of reduction/emergence 180 degrees