if theme can not override format_topics_renderer because of the extra parent, it is a serious problem. And if it is the case we need to raise a tracker issue for that and move discussion there. I have not tried it myself yet so I can't confirm at the moment.
The solution that I see is to make the format_section_renderer_base not abstract and other renderers such as format_topics_renderer will do $this->baserenderer = $this->page->get_renderer(...) in their constructor and instead of calling parent::xxx() will do $this->baserenderer->xxx().
And format_section_renderer_base::section_edit_controls() can be moved to format_base class. Actually I should have done it in 2.4 when I created format_base class. I only moved is_section_current() at that moment