As I understand it, your example should be the other way around. WhatsApp will need to offer a public API to allow Signal to send and receive messages to/from WhatsApp users.
Signal is unlikely to be deemed a gatekeeper, so can keep their closed communication ecosystem. They can just optionally choose to support interop with WhatsApp. If they prefer, they can also have big warning signs in the UI, when their users decide to utilize that interop.
As I understand it, your example should be the other way around. WhatsApp will need to offer a public API to allow Signal to send and receive messages to/from WhatsApp users.
Signal is unlikely to be deemed a gatekeeper, so can keep their closed communication ecosystem. They can just optionally choose to support interop with WhatsApp. If they prefer, they can also have big warning signs in the UI, when their users decide to utilize that interop.
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