"Now fast forward 80 years...this system that appears simple could be more complex than the current one."
I think you are ignoring one very important factor. Most of the complexity in the current system is in the area of deductions. Holding periods for computing gains & losses, carry-forwards for all types of deductions, ceiling and floors ...... virtually all of the complexity is in the area of deductions. If you scrap the whole concept of taxable income, you greatly limit congress's discretion to complicate.
If you had said that there is a risk that congress will complicate a well thought out plan and compromise its simplicity, which is one of its strengths, my response is that I agree, which is why we have to be vigilant and make sure congress doesn't pass some mutant version of the FairTax bill and call it the FairTax.
However, when you say that this proposal could end up becoming as complex as the current system, that is going a bit far IMHO.
Man did you ever say a mouthful! 90+ years and umpteen jillion pages of code and regulations and STILL we have no precise defination of the word "income". THAT is a problem.