The fundamental flaw in the guy's argument is that we don't create constants like π, e or what ever because they make formulas easy.

We make constants to represent fundamental properties, π is a fundamental property of a circle (C/D) and therefore is correct by definition.

Defining ππ or τ or a homer to equal 2π is just part of your working, however they are not fundamental properties of the circle and are therefore not of any interest in pure maths or physics.