Telegram already has two different group categories: Normal, an super groups. Telegram bumps 200 member groups to super groups. Signal's group E2EE scales up to 1000 members (hard limit). It's both doable, and scalable far beyond group sizes that have reasonable expectation of privacy.
I.e. they could drop end-to-end encryption after the group grows too large, e.g. after 200, 500 or 1000 members.
Public IRC-channel groups don't need E2EE. The overwhelming majority of groups are small, and would benefit tremendously from E2EE.
As for syncing messages between devices, it's doable, as shown by Signal, Wire etc.
Of course it's doable, but citing Signal for it ... you don't get old messages, only new ones. Might be a security "feature" but then again, let me export/import the history manually.
I.e. they could drop end-to-end encryption after the group grows too large, e.g. after 200, 500 or 1000 members.
Public IRC-channel groups don't need E2EE. The overwhelming majority of groups are small, and would benefit tremendously from E2EE.
As for syncing messages between devices, it's doable, as shown by Signal, Wire etc.