Technically, Obsidian is just a fancy 'browser/editor' for markdown files laying around. Should Obsidian disappear as a functional tool tomorrow, recreating the basic functionality (show, edit, manage links, follow links) would take a sufficiently motivated guy a few hours. If you need the 'petri dish' view, maybe a little longer. In such an event, I suppose enough people would be eager to build their own and OSS variants would emerge within days.

If you can stomach not working in Markdown, emacs' ORGmode exists and has all the functionality Obsidian has, and then some, open source, with a slightly different hypertext format.

Unless you are hellbent on one particular Obsidian plugin, you should be good.