In some cases, an ability to have multiple independent accounts/identities (pseudonymity) would - unfortunately but practically - beat true cryptographic security that Signal offers. I mean, personally, I'm less concerned about platform (e.g. Wire or Whatsapp) or some government agency learning that I'm talking to my buddies at certain schedule, than mixing up my acquaintances from different groups together, having to maintain a single identity for them all. Some people I talked with didn't knew my name or phone number, and I would be uncomfortable if they would. For me, in my life I've said less things I wouldn't want governments to learn about, than times I've used a pseudonym/throwaway account to talk to people.

This loss of user data is not advertised well enough up front, and leaves users feeling tricked. In many contexts loss of user data is an even bigger sin than weak security.

So you consider accounts not tied to a phone number "Slack ergonomics"? Before WhatsApp that was the default.