I don't see how the small teams (the ones being affected by the policy) would ever sign off on that.
Look at TJ Zizzo last year. They had the money to go to either Brainerd or Indy, but not both. Normally you'd always go to Indy, right? The US Nationals, biggest race of the year, biggest audience of the year, biggest purse of the year, who in their right mind would go to little old Brainerd instead of Indy? Well, they had a sponsor based in Minnesota, they had planned on having people come out to the Brainerd race, it was really important to them to have the car out on track in their home market, so on so forth. And if you know the first thing about sponsorship in racing, you know that if your sponsor asks you to do something, you do that something, or you don't have that sponsor for very long. So Zizzo ends up skipping Indy to go to Brainerd.
Had that rule been in place, his option to do that might not have been there, which likely ends up costing TJ a sponsor. While yes, Indy probably isn't going to be short on cars, there are all sorts of other examples like this where a rule like that ends up putting the teams in a box. Might be a good idea on paper, but in the end it's probably best to just let the teams do what's in their best interest, and accept that the occasional short field has always been and will always be a part of drag racing.