In 2019, the total value of the shadow economy in Italy amounted to 183 billion euros.
www.statista.com
These are the official numbers but as we know there's lots of dealings with mafia and a lot of stuff that cannot be accounted for.
I mean, being from a similar country the options are either 100 euros cash or 150 euros with a receipt. Everyone opts for the untaxed cash-option in such a case.
So the taxable figures could potentially increase (the 150-200bn ones) which is like 30 billion in tax revenue that was evading the authorities, so obviously it'll help. If somehow this trickles into some of the mafia shit getting reported the amounts could rise significantly.
But still, the point is that a flat income tax benefits the football clubs. And we should welcome this.