For a language to remain relevant in the long term, a system must be capable of evolving. Going out there and blessing potentially conceptually incorrect packages such as Mio is therefore not a good idea. The notion of "platforms" best resides in userland, where collections can compete and evolve.
By keeping platforms / package collections in userland we can get organic collections of stuff that make sense to bundle together. Imagine a "server platform", an "event loop platform", a "calculus platform", a "programming language platform". It would be detrimental to creativity to have all these collections live in the shadow of the "rust committee blessed platform".
But so yeah, I'm not opposed to the idea of platforms - I just don't think blessing userland stuff from the top down is the right play in the long term.
Tl;Dr: package collections sound like a cool idea, but if you care about the long term, imposing them from the top down is bad