Why would Boeing's MCAS be preferable to a modified Feel and Centering Unit?
On the 737 NG, at high angles of attack, the nose of the plane would naturally pitch down, helping to recover from a stall and to increase airspeed.

The larger engine nacelles on the 737 MAX are located forward of the center of gravity, which means that at high angles of attack they are pushing the nose up. MCAS helps to push the nose down in this situation, similar to how the 737 NG would behave.

The added nose down trim has the side effect of requiring more elevator input at high angles of attack, but that was not the primary purpose.

There are several reasons that the Feel and Centering Unit would have a difficult time providing similar functionality:


The airplane should pitch down even the absence of control input, so it could not be done by simply changing the way the elevators respond to pilot input.
MCAS takes input from angle of attack, altitude, flap position, and airspeed. The Feel and Centering Unit currently just senses airspeed, so it would somehow need to get the other inputs.
The Feel and Centering Unit is a mechanical computer, so adding new inputs and changing the behavior could be very complex to design and certify.


Another possibility would be to use the Mach Trim system to adjust the Feel and Centering Unit. The Mach Trim system uses the flight computer to adjust the elevator neutral position to provide stability at higher speeds. While the flight computer should have all of the information needed, stabilizer trim provides much more control authority.