One of the big issues I see with that proposal is that at some point, due to inflation an annual income in the ball park of 5 million dollars a year will be considered 'middle class'. And then the average person will be hit with substantial taxes from a then archaic tax law.
At that point you'll need a constitutional amendment to shift the goal post which is nearly impossible to do such. (At least at this moment in time it is.)
At that point the budget would likely have finally balanced itself out due to the higher tax rates. So
of course there will probably be politicians arguing against making this change as it will effectively kill the federal budget, because shifting the tax laws will mean the government will be taking in substantially less.
It's a super well intentioned idea at the moment but it's really a disaster in the making for a future generation.