Open plugins are meaningless when the platform itslelf isn't open. There are also lots of markdown centric note software so its not too much of a draw for obsidian. Personally I'd never use a proprietary note app, having to switch if the company shutters is too much, at least FOSS apps can be forked. Though, I've not heard anything bad about the obsidian team, maybe they open source it if they shut down. They also don't seem like the type to rugpull their customers. But you know how many companies people have said that about.

Gratis vs libre.

You openly acknowledged that Obsidian isn't open source. There's lots of open source software for Windows and macOS, yet neither Windows nor macOS are open source. Strange counter-argument in which you proved my point.

Name these FOSS apps that use custom formats. There's so many Obsidian-likes that are not only FOSS, but also use non-proprietary flat markdown files. It sounds like you're drinking too much of the Obsidian community Kool-Aid and trying to convince yourself of these things that simply aren't true.