> Would it have been impossible to nudge Node.js in the direction of where Deno is today?

Yes, do you not remember the whole nodejs vs. IOjs split? The community is already pulling node in tons of different directions.

> Would it have been impossible to replace Babel with a Go implementation?

Again, yes it would be impossible by dint of the fact babel is made to be a framework where you process JS ASTs using JavaScript. Rewriting it in Go removes the whole ecosystem of JS plugins it built, which is kind of the entire point of the project. A Go rewrite of certain transforms is a new project.