It's not that it's imperfect, it's that it's insignificant.
If the proposal had no costs, it would obviously be worth pursuing.
But it's a historically disruptive proposal. For changes on that scale, we should be targeting significant fixes. 10% is just nibbling at the edge of the problem --- to put it another way, if the cost savings is 10%, then a few years after the change was enacted, we'd be right back where we are now.
You can't iterate on restructuring the economy; it's not a PHP program. You can iterate on incremental fixes, but that's not what M4A is.
Don't let perfect be the enemy of improvement.
Constant improvement is what the system needs, and anything that improves it is good and should be done.