It works if you always want b to be the value deriving from a. However in the example above, we want the value of b to be temporarily out of sync of a.

My mental model has always been that a reactive declaration like $: b = a * 2 defines b in it's entirety ("Here's my recipe for b and all the ingredients, please Svelte make b always up to date"). And outside of this declaration b is immutable.

First of all, here is what I meant by updating reactive declared variable

The intention of the issue #2444 was to propagate the changes of the reactive declared variables back to its dependencies. in the context of this example, would meant, updating b should update a as well.

You declare a reactive variable and it's dependencies ("recipe").