My argument is more along the lines of: modularity is the (only) way to reduce complexity. We already have modular runtimes in other languages (project loom in Java, webassembly etc). Most people should not care about runtimes much. The ecosystem cost of async ended up being high. Thus, runtimes should be an implementation detail for most users.

Doesn’t mean Rome has to be rebuilt. Perhaps the async we have can be saved, but even so it involves biting the apple of actually defining precisely what a runtime is so that crate authors can think of them just like they think of allocators today (ie not at all).