Given that widgets are likely to eventually be replaced by blocks are sidebars likely to become a thing of the past also? Or will they be repurposed as block containers / container blocks? I noticed they weren't included in the mockups of the customizer side of Gutenberg shown at loopconf.

I think blocks will become available as widgets for existing/legacy themes. But going forward legacy widgets will be made available in blocks (see #4770), and in the future the “sidebar” would just be a group of nested blocks when themes are built with blocks from the ground up. See #4770.
Yes, with basic layout capabilities like columns or grid areas, a sidebar is just a designated area where nested blocks can be grouped. The mechanics to support that are already in place with the layout attribute.
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I think blocks will become available as widgets for existing/legacy themes. But going forward legacy widgets will be made available in blocks (see #4770), and in the future the “sidebar” would just be a group of nested blocks when themes are built with blocks from the ground up. See #4770.