You already do that with the AMT calculation.
But you only HAVE to calculate this twice if you care a lot about lowering your taxes. There are a LOT of people who do the standard deduction and never bother to figure out if the itemized deductions would be better for them.
And most people who make enough money for it to matter are likely using a tax program by now; they are cheap and do all the work, and will easily be able to add this calculation.
And if you do it on your own, the new calculation looks like it might be easy enough. I assume there will still be some deductions, but maybe not. If no deductions, it’s trivial — you take your gross income, subtract $12,500 for each dependent, and multiply hte result by 0.2.
The advantage is that it gives you the choice of simplification if you WANT to. But I don’t know if there is much of an “advantage”. It seems it would reduce tax revenue, since as you note people would use whatever gave them the lower tax.
But until we see details, we don’t know exactly how it works. I tend to like optional things, so if I want to keep my existing tax code, I can, just as I wish I could keep my existing health care.
The point is a parallel tax system is asinine.