I think the important point here is that these characters were not intended to be used as sub/superscripts for math & related fields. They are not a replacement for typesetting markup.
Also, arguably, in a_i and a(i) the i is not different per se, just that it is in a different place.
I don’t know if there is a consistent place to stop if Unicode starts doing this. Why not a_\breve{o}?