Retaining at least one successful build in Bamboo
IMHO if your successful build doesn't push to an artifact repository, there's a problem in your pipeline.

Jenkins or Bamboo do not aim at keeping built artifacts forever and they don't give you any versioning delivery, as such that's a point of failure in your delivery system. 

I'd advise to review your method to use an artifact repository and that will solve this XY problem of keeping a successful build inside the build system used as an artefact source.

Note: I can't speak for Bamboo's team, but adding this capacity involves raising the code complexity for a 'side usage'. As such it can makes sense Bamboo's team avoid it to avoid adding code complexity for a feature not in their overall goal. I don't think they avoid it to enforce a best practice, but more to avoid raising unnecessary (from the product point of view) complexity to their code base.