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If the company is bought by another company in part or in full then the original entity no longer exists and the patents enter the public domain.
If the company splits then the original entity no longer exists and the patents enter the public domain for all to use. Maybe the patent portfolio is split between relevant inventor departments during the process of divy-ing up resources. The requirement is that the patents remain with the originating entity as long as it exists; they are not transferable assets to be shuffled about on the chess board.
No more buying and hording of patent war chests. The amount of resources pissed away in this childish game