I'm in favour of authors extending existing core blocks - it's when the block doesn't exist initially that the problems starts (and then everyone writes accordion or tab or map blocks and puts them into block libraries with 30 more you never wanted...).

I'd like to see any theme author who writes a block adding the basic elements of that block as a separate block/plugin for all to use, then using their theme to extend it the way they want. That way you only lose the theme-specific extensions and styling if you deactivate the theme, not the whole block (which would presumably default to a Paragraph?)