As mentioned, in a large bus system, the logistical challenges of doubling the number of buses would be formidable, and would likely lead to overall service degradation.
Are people paying for the _seats_, here, though, or for the more direct route? (AIUI it's a hybrid bus-taxi sort of thing where it only stops where people have indicated they want to be dropped off/picked up on an app). Like, if it was all-seats but operated on a normal bus route, I doubt people would pay for it.
I'd also question how sustainable it is; as far as I can see it's basically an experiment by VW, and likely a subsidised one. It feels like one of these things being prepped in case self-driving vehicles ever work properly. And in any case the scale is _tiny_; apparently a ridership of 11 million ever.