These are the slides for Koichi Sasada’s RubyConf 2018 (last week) talk updating the community on his progress in the design and implementation of Guilds.
Hi, I'm the author, I'm also a Ruby core committer, and yes I'm aware of Guilds. The are a model for parallelism and don't really affect concurrency at all. At best, they provide a 3rd option on top of processes/threads/guilds.
Guilds felt omitted in your post since they (should) address one of the points you make about the usability/ergonomics of existing Ruby APIs for managing non-sequential execution.
But it’s definitely a bit early to tell what Guilds will actually look like as a final product.