I think the main goal of replacing ranked spells rather than adding to them was to reduce bloat in the skill list. On a narrative level, you learn a more powerful version of the spell but it takes more out of you and that's now the base level you cast the spell at when you use it.
I see what you're saying, but if we changed how Open Domain works with ranks we'd have to do it with every spell to avoid confusion. And that would be a balancing nightmare.