Yes; fair, essentially, but out of whack, obviously. Again, it applies to all people equally, under law. The problem is only that it treats different classes of income (Labor, Capital) differently, which needs fixing, IMO.
Wait...you just said that it is because the current tax system was progressive is what makes it fair. Then you said the problem is that it treats different classes of income differently, which needs fixing.
So which is it?
Are you talking about a flat tax?
No. I am not talking about flat tax. When speaking of current tax code, I assume folks are talking Income tax, and not cigarette taxes, tarriff / duties, etc. etc.
And while there are obvious flaws in many, including brackets and percentages, all are essentially fair (and thus legal) since the income and activity is taxed, and not the individual. So for example, Bill Gates and I have the same code. However, Bill has more Income and other activities, which are taxed, and Bill knew going in that the income and activities were subject to tax. But at any moment, Bill & Melinda could lower their household income to around $35 grand and not pay taxes. Thus, if it's what matters in the minds of some, it's essentailly fair, even is as I believe, grossly flawed.
Hopefully that helps, since I'm all in on it. Not sure what I can add to make it any easier to understand.