With that said, I was really kinda upset that they enabled "Delete Merged Branch" by default in the last major release (9.2): it really caused a few days of headaches of reverting development/staging branches that developers and merge approvers approved.
I ended up writing a small Chrome plugin to uncheck the box, and distributed it to the developers that I work with before going in and learning enough angular to disable the box from being checked by default.
Sid; I have nothing but respect for you and the rest of gitlab, but, please don't change the behavior of how gitlab works (and yes, I understand convention over configuration) without giving either project or site administrators the ability to disable those changes.