Why dyn-star (dyn*)?

I've read some texts about dyn* this post for example by Niko Matsakis.

dyn* is supposed to be a two-word value, where one word is vtable, and another word is data (Box<Future> or just Future if the Future is not larger than one word).

The problem dyn* is solving is real, but should dyn* be built into language?

Because as far as I see it can be made in a library.

Here I have an implementation of "dyn*" for Display without any support in the language: play.

The largest issue with this implementation is that currently in rust it is not possible to parameterize code by traits (i.e. it is not possible to generalize DynStarDisplay to handle arbitrary traits, and would be great if Rust could do it).

If this is done, dyn* may be not as useful, or be made syntax sugar for type DynStar<Trait>.

What am I missing?