I believe this is the crux of it. Sure if you have a fixed budget and team size then you're naturally going to get a better result focusing on one platform rather than 3, that's just common sense.
But that in no way equates to a rule (that some people suggest exists) that the highest tier graphics will always come from exclusive devs. As you say, simply increasing budget (team size) and/or time on a multi platform game can produce the same results. And since multi platform games have larger markets and thus potentially larger revenue they can also potentially afford the larger budget without negatively impacting profit.
Or they are basically the same machine with one having a little more horsepower to the tune of around the different in those two resolutions. It's not like you can do some particularly clever tricks with the Jaguar or GCN architecture with one console that you can't do with the other (okay perhaps the esram does mix things up a little there).
