I don't know your specifics, but I have worked on some large scale architecture changes, and 200 engineers + 2 year feature freeze is generally not a reasonable

I don't know your specifics, but I have worked on some large scale architecture changes, and 200 engineers + 2 year feature freeze is generally not a reasonable ask. In practice you need to find an incremental path with validation and course correction along the way to limit the amount of concurrent change in flight at any moment. If you don't do this run a very high risk of the entire initiative collapsing under its own weight. Assuming your estimation is more or less correct and it really is a 400 eng-year project, then you also need political capital as well as technical leadership to make it happen. There are lots of companies where a smart engineer can see a potential path out of a local maximum, but the org structure and lack of technical leadership in the highest ranks means that the problem is effectively intractable.