From what I've read[1], vim9script was pushed and developed almost exclusively by Bram. With him, a lot of knowledge about its internals and vision for its future dies.
You're correct it was very much a "Bram project", but that doesn't mean the language needs to die with him: other people can work on it (and already have!) Vim9Script is also "finished", more or less, as "finished" as languages get anyway. The features Yegappan mentions are what we might call "optional features".