Relative positioning is the same as the default static positioning, except you can use offset values (top, left, etc.).
Relative seems objectively more powerful that static positioning.
Is there any disadvantage to resetting in CSS all elements to be relative (and overriding for absolute as required)?
Absolute positioned elements are relative to the closest relative container. If you set everything to relative then it will be a PITA to use absolute positioning in cases where you want your absolute element to be positioned relative to an ancestor further up the tree, because the parent of the absolute element will always be relative.