Why would fish cells die when fish dies?

“seflish gene” view of evolution predicts we’d see fish dissolve into individual cells when fish heartbeat stops
1. Each fish cell individually benefits from detaching and existing as a single-celled plankton
2. Each fish cell has genes necessary to become fish sperm cell and potentially fertilize some random egg laying around
3. Just sitting there and allowing random organisms eat you seems hardly optimal for the cell
There are, in fact, multicellular organisms that do routinely dissolve into single cells
1. They are called facultative multicellulars
2. But they are always small and unsophisticated
3. Seems like to evolve a large complex multicellular body you need to abandon facultativity.
4. Why can’t facultatives make complex