For the usecase I see these to be useful, I don't see lack of e2e such a huge concern. I would assume that in most cases the server would be more or less trusted in the context they are used in, so TLS style client-server security would be adequate.

In other words, if I really wanted to have discussion secret enough to warrant e2e I probably wouldn't be using company/project chat for that no matter what sort of promises it gives.

That being said, of course e2e chat would be beneficial addition, and might help especially in larger enterprise deployments.