Obsidian runs on a local folder of files on your device, so there are a dozen different sync services you can use including several free options, e.g. Git, Syncthing, etc.

With Obsidian Sync, your $8 don't only go towards syncing. It helps Obsidian remain profitable and 100% user-supported (no VC investors). Subsidizing costs in pursuit of growth is what got Evernote here.