It's not just about learning a language. Given two equivalent languages, writing a project using one or the other is always going to be less work and less of a maintenance burden than writing it using both. A competent manager will take that into account when deciding what tools to use. On top of that, learning a new language has a cost. Of course Rust and JavaScript are not equivalent, but which one is 'better' is highly subjective and dependent on how you measure 'better'. So a manager needs to take that into account. But my fundamental point is that using two languages for a project adds overhead, and learning a language adds overhead, so unless cost (including time) is irrelevant, there must be a compelling reason to choose a dual-language solution* over a single-language solution, and to chose a solution that requires your devs to learn a new language over one that does not. Not to mention switching platforms has a massive cost if your project is already mature. Even if you're creating a new project, if your team already knows JavaScript and doesn't have any particular objection to Electron, there's no compelling reason.

If there is a good reason to learn a language then people will.

Sure. Except in my experience interviewing candidates and from what I've seen online, there are a lot of developers out there who aren't very good. I am not optimistic that the average developer will have an easy time learning a new language. If the "we" in "Is this the electron alternative we've been waiting for" is you and I, that's not a problem. But if OP meant to suggest there will be a large-scale shift away from Electron, then the average developer is quite relevant.

*As someone else pointed out, Dioxus is designed with the intent that you'll right the frontend in Rust, so it's not exactly dual-language like I thought.