where the endings “-tion” and “-tio” both were in use.
Alack, not so.
The Latin ending is nominative –tio, genitive –tionis, dative –tioni, accusative –tionem, ablative –tione.
That’s a pain, sure, but the common pattern is that the underlying ending seems to be –tion– (and that’s what you’d reconstruct the proto-Latin nominative as). The nominative drops the final –n since there’s nothing after it, since Latin doesn’t normally have final –n‘s; that’s what makes a lot of Latin declensions look inconsistent (and even more Greek ones).
When the Latin case system melted down, speakers went with the regularity they could see, and reduced it all to –tione (Italian) or –tion (Old French). English got its Latin from Old French. And it did not have the option of getting them without the final -n, straight from Latin.
Much more recent loans straight from Latin will take them as –tio ; the only one I can think of though is fellatio. And the taboo nature of it probably explains why it wasn’t Francified to *fellation: it was not so much a borrowing, as a way of hiding the meaning, so it wasn’t a real English word.
What would happen if you went back and switched –tion to –tio?
It’s an odd thing in English to have words ending in –o; that’s not an English thing, it’s an Italian and Spanish thing (see List words ending with io ). It’s an Italian and Spanish thing precisely because it’s not a French thing—French aggressively dropped final vowels (-tione > -tion; *bono > bon). You couldn’t switch tion to –tio in isolation back then; you’d have to make sure all those final vowels were kept in general, because if you didn’t, people would make –tio sound like something more familiar. Like say –ty (which comes from Latin –tia).
To make that work in turn, you’d basically have to have English borrow words from Italian or Spanish instead of French. Words like braggadocio and machismo would be closer to the norm than the exception. And as an added bonus, English would sound even less Germanic.
Opera singers would be grateful.