This is impossible in Obsidian, and one of the first principles of the developers (and the app). See the “Forever” heading on the front page.
Because Obsidian uses your local files, you will always own your notes. You can open the .md files created by Obsidian in any other Markdown-editing app, including many free alternatives. Other Personal Knowledge Management (PKM) apps also use similar syntaxes as Obsidian, so you can even transfer functionality like backlinks between apps.
As for the business model/pay structure, Personal licenses are $0 forever: https://obsidian.md/pricing
Edit: oh, one more thing. There are similarities between Obsidian and Roam, yes, but as is often the case in technology, Roam is not the first to implement many of its key features either. What I think Roam sells is actually a workflow: its designers and developers have done an excellent job of socializing PKM workflows and using Roam’s features to teach people a new way of working. But that way of working is not exclusive to Roam—its roots lie in the analog form of index cards and boxes to put them in.