It requires me to use git-diff. How do I do it without involving the VCS tool? How do I get both files side-by-side in Emacs so I can go through them thoroughly (some of us prefer that to reading diffs, you know)?

Since you can reconstruct deleted branches, I don't see the problem with deleting them.

I want the history of my codebase to be easy to get a feel for; that means major work (e.g., a long-lived branch which made significant changes) should be easily accessible. Telling people to hunt down the right merge commit and work back through its ancestry doesn't do that. Inventing ad-hoc schemes of tags or renamed branches to try to make up for the inability to keep a branch but say you're done with it doesn't do that.

It reminded me of why I like git so much.

Meanwhile, it's (one reason) why I don't like git so much.