Everyone thinks we don't need another one until they see one that does something they like but hadn't realised would be useful. WhatsApp with its free messages, Twitter with its enforced terseness, Snapchat with its take on ephemerality - they were all adopted by millions of users who, before seeing the app, would probably have declared they didn't need a new messaging app. Yet they installed it, tried it, and continued to use it.
People will always want to try new ways to message one another. Consequently we'll always get new messaging apps.
Twitter's success wasn't due to "enforced terseness", it was due to the fact that it supported posting to it from SMS messages back when the majority of the population still had dumbphones. Once smartphones became ubiquitous, Twitter kept riding that wave due to network effects. In fact, the entire reason for the 140-character limit was so that it could interoperate with SMS cleanly.
I do somewhat agree with your overall point though. And it's worth noting that sometimes that "new feature" can be things like "my parents don't use it yet", in the case of teenagers and the like.
I agree. But what's the feature here? I lament the audio quality of cell phone calls, but audio quality depends on bandwidth, and this requires an internet connection, which is going to be spotty depending on location and when transitioning from data to wifi. Every feature here seems a marginal improvement on what already exists (audio quality, UI, sync, security, battery use). No new concepts. Don't get me wrong, I'm happy to use it, but I don't see an impetus for mass adoption.
My understanding is that it has inline content like Whatsapp and desktop support & sync features like Skype (Skype took a lot to make the sync at least tolerable - but I guess they already know what they're doing). The supposedly high quality of the calls and inline 3rd party content support (soundcloud and youtube for now, I suppose) are the bonus features.
People will always want to try new ways to message one another. Consequently we'll always get new messaging apps.