As they say, the best diagram language is the one your tool supports; and by that metric Mermaid and graphviz seem to be winning (depending on which ecosystem you are in, Mermaid seems to have the edge in the Markdown world, graphviz everywhere else).
D2 looks like a nice upgrade, but to be as useful as e.g. Mermaid a bunch of plugins will need to be written.
yup agreed. we prioritized IDE plugins, and have a native plugin system, recognizing the importance of ecosystem. e.g. a proof of concept plugin to make diagrams look hand-drawn: https://github.com/terrastruct/d2/pull/91
D2 looks like a nice upgrade, but to be as useful as e.g. Mermaid a bunch of plugins will need to be written.