Great article! Covering the opting out mecahism and the rationale behind it is very helpful with such a visible UX change. Thak you!

If you start using the new block-based widgets system, you will lose all of your widget blocks upon activating the plugin. There is no going back, so be sure this is what you want. The former blocks will not reappear if you change your mind and deactivate Classic Widgets.

I think this was a bug, it's definitely not the intended behaviour. Opting out of the block based Widgtes Editor and then opting back in should preserve all data, no matter how many times one does it :)