it made more sense when mobile phones operated via SMS rather than 3g/4g/wifi, but it doesn't make a lot of sense now to have the character limit. What do you actually gain by capping posts to 240 characters per message? People seem to primarily use Twitter to communicate and forcing people to break a thought into multiple posts adds nothing to the experience.

"Twitter is a telegraph office" -- but should it be?

Look at ATOM/RSS, they are telegraph offices too, yet they don't have character limits. ActivityPub services like Pleroma also are "telegraph offices" without strict character limits yet they don't seem to suffer for it. In fact it's much more pleasant to use Pleroma for this reason, you have the freedom to express complete thoughts.