How is managing Bamboo different from managing Jenkins or TeamCity? And why is Bamboo overkill?
I mean, I understand the argument that it would be nice to have Pipelines in Bitbucket Server, which would make it more comparable to self-hosted GitLab (which includes GitLab Pipelines). However, that same thing can be said of Jira & Confluence, as those are also features that come out of the box for GitLab. The difference in value proposition between Atlassian and GitLab is the fact that Atlassian recognises that each of these tools have their own set of rich features that warrant a stand-alone product.
I would pick the Atlassian stack (Jira, Confluence, Bitbucket Server, Bamboo) any day over only using a single GitLab or GitHub solution for VCS, project management, wiki and CI/CD.
Bitbucket Pipelines for Cloud only exists because of the strong competition in cloud-based CI/CD solutions. In the on-prem world, if you have already chosen Bitbucket Server over GitLab, you will probably also have Jira & Confluence on-prem. Adding Bamboo is not such a big deal.