If something isn't a great idea, the tools should support that approach

I assume that soon developers will publish more components for svelte, and any possible class conflict is problematic. Yeah, it's rare case, but it is.

There are work arounds, but nothing clean. I just feel like this should be functionality that should be part of the slot feature.

I totally get not wanting to extend the syntax. I tried doing these things and in practice it was not easy or pretty. Actions provide a much cleaner and easier way to accomplish a certain set of functionality that would be much more difficult without it.

Actions add additional functionality to elements within your component's template that may be difficult to add with other mechanisms. Examples of functionality which actions makes trivial to attach are:

If this was tied into Svelte's flow with hooks this would not be necessary since it would know when it was being removed from the DOM.