I don't think your example illustrates an actual security issue, and I don't think it's useful to users to expect EV certificates to change how the same origin

I don't think your example illustrates an actual security issue, and I don't think it's useful to users to expect EV certificates to change how the same origin policy works. Personally, as a user, the value of EV certificates was not that I "didn't trust DV certificates", but that (at their best) EV certificates validated the link between a known corporate entity and a domain name. Once I know that "som.example" and "Somex Ample" are the same entity, there's no reason to worry about "downgrades" or not trust DV certificates for the same domain name.