This doesn't answer my question in the slightest. What does it offer so nice that vscodium doesn't have and that would push me to switch to theia?

As an outsider to the conversation I'd say it answers it vaguely at least.

Perhaps you could ask a question less vague than the reply it incited? What is it that you want that keeps you with vscodium over other options? Or if you don't have enough information/experience of them, just what do you like about vscodium enough that you aren't particularly feeling any need to consider alternatives?

- Certain core, built-in plugins do still contact MS servers.

- Vscode has limited what extensions can do on the platform. MS own extensions can do things that others cannot.

- Theia extensions can do what oss vscode extensions cannot. Theia supports all the non-ms specific extension API's so it is fully compatible with all/most vscode extensions. But Theia goes further. Part of Theia's vision is that you could ship your own Theia, the whole platform is open.

So, Theia is a safer bet. There is no rug to pull.

Also, think about chrome <-> chromium. When Google pushes for a new API that limits what ad-blockers can do, then Chromium has a problem. Because then it becomes a fork, as upstream becomes incompatible with chromium.