Capone got nailed for evasion because his brother got nailed for evasion and he got scared so he tried to preemptively normalize his own tax situation but in doing so indirectly admitted that he hadn't paid taxes, so they busted him with no further evidence required.

But they also were trying to bust him for evasion because he declared zero income and lived a visibly lavish lifestyle. The government was gathering evidence on his spending to estimate his income and how much he evaded.

You can't sustainably solve that problem by simply paying lots of tax on magic illegal money, especially not in today's interconnected world. Laundering is core to the solution.