There is no backwards compatibility issue with added behaviour.
No? Scripts use on open for query flood protection.
And setting on open to trigger when you open a query isn't going to affect a query flood protection script. It's not like you're going to open enough queries yourself fast enough to trigger that kind of script.
I really doubt there's any backward compatibility issues with triggering on open when you open the query. If there are, it likely means a script is so badly written that it needs to be updated anyhow.
Also, your query popups isn't what argv0 was referring to. That still uses /query, so it *is* covered by the alias shown. argv0 was referring to things that the alias wouldn't cover.