I don't disagree with anything you've said but I would personally take this line of argument a step further. I think Bram learned from the neovim fork. Neovim bolstered the existing lua support by reducing the impedance mismatch between the lua language and the vim host. A lot of people enjoy writing lua more than vimscript but the value-add hasn't proved compelling enough for people who didn't mind vimscript. The official neovim repo still has twice as much vimscript code as it has lua code. Even if you claim vimscript is 2x as verbose that would still leave them on equal footing within the project that is putting lua forth as an equal competitor.
My impression of vim9script is that Bram had a list of goals (clearly outlined in the docs) and saw the typescript project as a model to follow. The docs themselves mention typescript as a source of inspiration for some features. The typescript project had a similar set of goals and managed to gain massive adoption both among people who had previously disliked javascript (due to the improved semantics) and people who had previously like javascript (due to the speed improvements, transpiler ergonomics, etc).