First, getting people to pay any money is such a massive barrier to entry that it limits your audience by orders of magnitude. WhatsApp (pre-FB) is a rare outlier. Likely the only reason this worked is there simply weren't any other messaging apps for many of their platforms at the time and SMS prices were still outrageously high. none of that is true anymore.
Second, just the name ("HalloApp"). I mean the fact that it's "Hallo" not "Hello" tells you a lot. This is developed on a budget. It's also too close to "WhatsApp" so (combined with the subscription) it just smacks of former WA engineers trying to create WA 2.0. 2.0s almost never succeed (MariaDB being a rare exception).
Third, it's not really clear to me what the messaging model here is from just reading this. How is this going to be different from the user's perspective compared to WA, Messenger or whatever?
Fourth, any new platform that is similar to another is simply going to be a haven for those that are banned on the original platform or who otherwise feel they proselytize whatever whacko positions they hold. This is not the basis for a healthy platform.
Lastly, people. The dirty secret of social media is that the core problem is people. On anonymous online spaces people are hateful and vindictive. They believe crazy shit and want everyone to know about it.