I think that perception you refer to has to translate into dollars but that's JMO.
Current conference members have been receptive to teams that will increase total revenue AT LEAST to the point it doesn't reduce their current share. If they bring in a new team that doesn't increase total revenue enough and decreases their share when divided equally, then it is a no go. I believe some have brought in new members with the agreement the new members wouldn't get a true equal share until a certain point in time.
But the best thing ISU has going for it is their AAU status. Like many others have pointed out, the real money is in research dollars. If adding ISU helps in that regard, then the current members would probably accept them whether their athletic program is good enough or not. Rutgers was like that but Rutgers also had the Eastern seaboard television sets. ISU doesn'thave that number.