There's no inherent reason that the whole life cycle can't be handled in a single tool. However, there have been tens of thousands of person-years spent on these tools, so people here are pointing out that it is a tall ask for any company to create one tool that integrates everything. This goes doubly so if it is only going to be a side project to GitLab itself.
We'll only get there if Meltano get a lot community contributions. We think there is space for an open source end-to-end product that works out of the box. The contributions will tell if we're right.
