Yeah I hear you - there were some nice things about widget extensibility APIs.
But the flip side is that it’s been incredibly difficult for us to modify ANY of the widget-based UI in core, because we have no idea what random methods/decorations people might be introducing. That’s why widget customisations have seemed relatively stable - we’ve been too scared to touch the core implementations.
Our solution for this going forward is Wrapper Plugin Outlets. These allow themes and plugins to optionally override very small chunks of templates with their own implementation.
For example, see how Chat conditionally overrides the home-logo with a custom component. That works for the existing widget-based header, and the new glimmer-based header (coming soon! 
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We’re generally happy to accept PRs to add new wrapper outlets in various places. If you’re unsure about a particular use-case, please feel free to open a dev topic with details!