No, it will not be a case.
In any R&DD organization, there are less than 20% of capable people and 80% of just "warm bodies." Consequently, making good offers to the 20% best, "keeping" offers to the next 40-50%, and getting rid off ~30% least productive people makes a lot of sense.
Finally, motivated individuals to start their own businesses will do it regardless of Genentech staying independent or being consumed by Roche.
Based on my own experience, in situations like DNA/Roche, companies can make "too good to reject" offers (30-50% raises together with up to 1-3 years annual pay signing up bonuses).