But the question is how do you reduce? How do you decide who to keep, when whittling down to ~100 from 4000 (or whatever the current number is)?

From my experience, this type of re-org leaves the resultant company unsustainably top-heavy. I recently left a company who downsized but refused to hire anyone "junior level" because their pivot "requires all stars". And we all know, when everyone is a senior, no one is...