my question is why is it profitable flying at speeds so high that exceed mcrit increasing drag?
You appear to be assuming that we will minimize our fuel costs if we fly at the speed at which fuel flow to the engines in kgs per hour is lowest. This is not true.
As an example let’s assume that we have the following:
Fuel consumption rate at Vmd = 1000 kgs per hour.
TAS at Vmd = 250 knots
Fuel flow at Vmrc = 1200 kgs per hour
TAS at Vmrc = 330 kgs per hour
If we fly 1000 nautical miles form A to B at Vmd we have
Flight time = 1000 Nm / 250 Nm/hr = 4 hours
Fuel consumed = 4 hours at 1000 kgs per hour = 4000 kgs.
If we fly the same 1000 Nm from A to B at Vmrc we have
Flight time = 1000 Nm / 330 NM.hr = 3.03 hours. (To 2 decimal places)
Fuel used = 3.03 hours x 1200 kgs per hour = 3636 kgs
By flying at Vmrc which is 80 knots faster than Vmd, we have used 364 kgs less fuel.
When considering the fuel cost over a given distance the important thing is not the number of kgs of fuel we burn per hour. It is the number of kgs of fuel we burn for each nautical mile. This is why get the maximum range and hence minimum fuel cost if we fly at the speed at which the TAS to Drag ratio is maximum. The fact that this speed is slightly higher than Mcrit is not important, because there is no significant drag rise at Mcrit.