I think that's a good idea and I wish they would do something like this. FWIW I disagree that the console is a usability disaster. We're very prone to hyperbole in this industry. It's definitely got a ton of room to improve but it's not the worst and it's usable. Hundreds of thousands of people use it all the time.
I can give some background that might be helpful on why the UIs might not be open sourced.
Each service team builds their own console. Most of the original AWS consoles were written in Java with GWT. Some eventually moved to Angular. There's a big push internally to use a more consistent and familiar UX experience across services. There's an internal UI/UX platform called Polaris (I'm not leaking anything here, you can see this in the source of some consoles and in the console.log statements that leak to prod) that most of the new services use.
Most new services are built with React + Polaris. The console API for many new services is not the same API that is exposed to the SDKs/CLIs. These APIs aren't public or documented. If they open source every console, and people build on that, they're tied to supporting those APIs in perpetuity. That slows their pace of iteration on the console.
If consoles are open source it could be another potential vector for service updates to leak through.
Another concern is that releasing the UI kit for the AWS console is going to make it way easier for phishers to build out convincing lures. TBH I'm not super concerned about that.
If I were AWS I'd open source the UI toolkit and give various service teams the option of open sourcing their console. From there I'd release a theme/color scheme for people who want to build extensions.
AWS is very customer feedback driven. However, most of the customers providing feedback are not the ones using the console day-in-day out. The people actually using the console are not the ones talking to AWS. If you hit a UX issue then use that feedback button on the bottom left of the console and let them know.