Why does email survive when platforms die?

Because of decentralization.

When platforms like MySpace, Vine, or Friendster sank beneath the waves, all communication tied to them vanished. Email, however, sails independently.

Your subscriber list is your property — not the platform's. If an ESP disappears, you simply export your data and move your "ship's log" to a new harbor. The system endures because it belongs to no one and serves everyone.