Rules can always be changed, providing there's a good enough reason to do so (see the example above about UEFA changing the rules to allow Liverpool into the Champions League next season). There was a need to determine which course of action would result in the least damage to F1 as a sport, and nobody was willing to do that. It seems that once again, the teams (and promotors) were only thinking about themselves.
By only having 6 cars out there, the result for F1 is far worse than putting in a chicane or allowing new tyres to be used. The North American market (where, let's be honest, Bernie and co. are looking for a lot of money to come from) is now going to be completely disillusioned with the sport and it's likely that this is the last Indianapolis GP we'll see for a while.