I disagree. The web app editor is closed source, but much of what it provides is open source so editing is a similar (and imo better) experience locally. The typst compiler and LSP and everything you need to use it is open source.
Imo the situation is more like if overleaf were also the people who made the LaTeX project originally.
I think the only possible issue with the typst org dying (assuming after the full 1.0 version so it's mostly maintenance) is that packages are automatically downloaded from the typst site, but an open repo can trivially be made considering that the set of packages used is just from a open source git repo and the closed source site just hosts tar.gz files of the folders in the repo. Not a big deal I think.
They have a deep incentive to drive users to subscribe, and that's directly at odds with keeping all of the document rendering open source. It makes a lot of sense for them to provide document features that are only available to subscribers.
What you suggest seems plausible, but there is a very good counter example. Overleaf is also managing well by relying on the open-source LaTEX. What drives people to subscribe is not the typesetting itself, but the ecosystem around it (collaborative editing, version management, easy sharing, etc.). You can make money with those and still have the rendering free/open-source. I believe a similar thing is/will be true for Typst as well.
That is a bad counterexample. There is a world of difference between the main devs offering a paid service and some unaffiliated company offering services.
In principle, having a reliable source of funding for typst is great. However, as a journal this would make me hesitant: what if down the road some essential features become subscription-only?
It helps that the LaTeX ecosystem is such a flaming dumpster fire that you all but need a tool like OverLeaf to use it effectively.
They have some incentive to drive users to subscribe, but they have other forms of income, and I think if they ever implemented even a single feature of actual rendering that was closed source their community would riot and we'd get a community managed fork (probably by the guy who does the language server...).
The only way they can continue to gain traction is if they never ever in any way lock people to the web app. Documents must be portable, it's part of why someone would want typst anyways.
I do not see a future where this happens, and if it does it will be because the typst org has changed hands and is also no longer particularly relevant to the future of typst the language.
Is there really a community of volunteer contributors that could fork it if that happened? Typically with a corporate-backed project like this, the corporate development tends to crowd out the formation of a volunteer community of contributors that would be able to take over development.
All the typesetting extensions and such are a community effort. There are so many specific use cases that can only be/will be done by very specialized academics that a non-networked product would die on the vine.
There is quite a clear distinction/border between an Input-Output rendering kind of program sitting beneath everything, and a web service providing stuff like collaborative editing, free hosting etc on top.