I wonder if it took so long because of how difficult pluralising-after-the-fact is [1]. For example, I imagine all client apps keep a list of contacts and a one-to-one public key for each contact (rather than one-to-many). [1]: https://www.swyx.io/preemptive-pluralization
Whatsapp breaking its one-phone assumption must have been hard, but they'd managed to do a web version, so some intermediate kludgy solution was probably feasible sooner. Maybe it was de-prioritised or decided against.