By using the built-in codeblocks functionality of Markdown, Mermaid allows you to source-control the same exact graphs that you would have normally put in as images.
So the desire "Turn this generated graph into a diagram for inclusion in this PDF report" would be deemed a category error: my desire is wrong.
It clearly makes some folks happy, but to me it's the antithesis of a "tool"; it's more a "website feature". Different strokes.
This way, there's no need to separately generate an image output to include into the document later on.
Different "tools".
That said, I'd love a dedicated application for diagraming, with exports to various formats for other use-cases.
For me, I'm often either (1) sharing diagrams outside of github (e.g. email), as well as (2) generating a bunch of diagrams based on code or whatever, rather than defining a diagram in a markdown file. There may be better tools for those usecases. Those were what I was trying to do last weekend, too.
But I've definitely passed on adding diagrams to my .md files, or sighed at needing to add yet another imgs/ dir and files to a repo folder. My toolbelt now has another thing hanging off it!