I feel it will be a game changer. Many people still think a Tesla is a $100k car, and many more people don't have $7500 laying around up front, or want to roll it into a loan. A Model Y that is $43k 'out the door' vs. waiting months and months and months and doing lots of paper work to get your money back later, this will be 10x more appealing to... literally everyone. I can't imagine anyone wanting to get a rebate when they can get a 'coupon'.
You can then directly compare a $43k Model Y to a $43k Honda or Toyota or Ford or whatever, and people will have the 'ahh hah' moment, and these things will sell like crazy.
but, it's just a hunch.