For AMT the taxable about will be higher of the following
Normal tax liability is levied on the normal applicable rate on taxable profit/Income.
OR
AMT is levied at the rate of 15% on Adjusted total income adding applicable cess and surcharge (health and education cess) and surcharge(SC).
But presumptive u/s 44AD says 6% will be considered as taxable profit and on top of that normal tax slab applies so if the normal tax comes below the 15% rate of Adjusted total income and people are bound to follow the AMT rule then that is the benefit of adopting Presumptive tax scheme?.