I agree there’s a concern with users migrating to a sub-par intermediate solution (adding constructor arguments), but I don’t see why trait parameters would be useless because something that they can replace (early initializers) is deemed to be. Trait parameters have a nice symmetry with class parameters, and thanks to some careful restrictions, they are less tricky to reason about in terms of initialization order.
With the introduction of trait parameters, the only conceptual distinction between a class and a trait is that a class fixes the linearisation order, while a trait does not. (Low-level differences remain due to platform restrictions, of course.)
Our take on early initializers (or anonymous traits, if you will) is that they are a niche feature, which just weighs down the language without much advantage (nor usage, crucially – as it would be too costly to remove commonly used features in terms of upgrade drag/pain).