For one, they can stop pretending that JS and the DOM are two completely separate entities, and lock the two working groups in a basement until they figure out a sane way to do reactivity, natively, that actually works.
Slots need to have a host element to select slotted nodes from. ShadowRoot provides that host. Everyone who I've seen say that it's easy comes up with an ambiguous system that doesn't work in many cases and could never be standardized.
Which is a common problem with most "web components suck" takes - they have no sympathy for standards developers or the difficulty of the problem and somehow assume the spec people are dumb or evil, and they don't do the work to show how something could realistically be better.
Stencil.js[0] has a working "proposal" or workaround. Not sure if it is good or bad but at least it seems to work. When you ask users of an API how it could be better they should not be responsible for the implementation. If it can't be done so it is better to not do it in place of having a half working solution that most people decide to not use because it does not work for their use cases.
> they have no sympathy for standards developers or the difficulty of the problem
The problem with the browsers API is a lack of an understanding on how to design good ergonomic APIs for the end user. All the standards developers I talked think shadowDOM is great and people that don't like it are holding it wrong.
In the real DOM you need to know which element from project _from_. The <slot> itself is only half the connection. In a light-DOM only document full of <slot>s you won't know which elements to project where.
> The problem with the browsers API is a lack of an understanding on how to design good ergonomic APIs for the end user.
This could be slightly true, but the statement also misunderstands the role and priorities of standards. When adding new capabilities to first priority is to make something possible at all, and coherent with the rest of the platform. Making a sugary high-level API is often left to libraries at first, and is exactly what libraries like Lit are.