I’m not fully for or against Obsidian open sourcing, but I didn’t really understand your argument.
Open source doesn’t necessarily mean the community contributes. There are many projects that maintain full control of their source but open it up so that the community can review it (personally, I work with a few in the CNCF space). Obsidian would be able to open source without opening up to more problems. They could make the code available and clearly state they do not take PRs. This would allow for security reviews and faster bug resolution since the community would be able to help find the root cause before even posting bug reports. This is how we work in the Kubernetes space. All the for-profit organizations in my work are open source and making bank (k8s is a pretty active niche, though, so other projects’ mileage may vary). The point is there are models that work, and I’m not sure I’ve seen an argument for staying closed that takes those models into account.