That's a feature, not a bug. If you want conversations to be searchable, use email. Instant messaging is like phone calls or in person communication: transient (if potentially async).
> No support for attachments.
Good, no silly gifs cluttering up the chat. But this is a feature that's easily added.
> Poor user authentication
This one probably has the most merit. But probably easy to implement with a few server tweaks.
> Short - and unpredictable - message length limits
Brevity is a feature. It's a chat, no one wants to listen to a monologue in conversation. The unpredictability is a bit annoying.
> Crude permissions system
How fine grained do you need it to be? Read and write access for channels exist. There are plenty of bots which can grant additional permissions based on whatever criteria you set out.
But hey, I'm also that stubborn bastard who still likes text-only email.