While this may be a better sandbox, actually having a separate computer dedicated to the task seems like a better solution still and you will get better performance.

Besides, prompt injection or simpler exploits should be addressed first than making a virtual computer in a browser and if you are simulating a whole computer you have a huge performance hit as another trade off.

On the other hand using the browser sandbox that also offers a UI / UX that the foundation models have in their apps would ease their own development time and be an easy win for them.