Ok, didn't realize he worked on it. Bundler is great so that gives me some hope for Yarn. But in general I think its more stable in the long-term for projects to use open standards instead of vendor solutions for browser package management.
I don't agree with any of your statements, and I think they are inaccurate.
jspm aims to be type-agnostic with respect to modules (supports all major types, hence why it is called a universal module loader). jspm certainly doesn't advocate rewriting your modules (you can use exports, AMD, ES6, whatever). In fact, when the browsers natively support module loading, systemJS will go away (which is really just a polyfill for this functionality).
You say "decent performance" and "predictability". What is the basis of this claim? I've heard these all before, but unless you've actually shipped a product using this tool I don't know how you can back this up.
Its disheartening to see so many duplicate efforts to solve the same problems everyone faces. Instead of supporting an existing open-source project that attempts to solve the problem in pretty much the same way (jspm), a conglomerate once again builds their own from scratch. I do not think the underlying motivations for doing so are questioned enough. Sure there is control, but it is of course anyway-you-slice-it an aggrandizement of their brand. We should be ashamed as a community to support the idea of yet ANOTHER client side dependency management system. How oh how did we ever solve this problem before Facebook came along and made Yarn in late 2016?