I'm pretty much in agreement with Richard and Eric on this statement - "Progress is a theme display".
If we rename blocks to, or create a new plugin called, "Theme display", that defines areas of the screen to drop them in, then sure. But otherwise, I don't think the theme should decide what functions and/or content get displayed.
I am unconvinced that we can do away with blocks as plugins. If we want to rename them as "widgets" or "tiles", fine. But they provide a way of creating add-on functional and content features to Moodle that I don't see any other way to provide. I am sure there are some blocks now that could be suitably replaced by one or more other plugin types, but not all of them.
Without the ability of customizing Moodle "pages" with block add-ons, Moodle would be a "one-size fits all" solution like Canvas. One of Moodle's primary strengths is the ability to customize and tailor it to specific needs.
mike