My understanding is that MCAS was added to make the airplane certifiable with the new engines.
To be more precise, this from Aviation Week:
The MCAS is a new flight-control-computer (FCC) function added to the MAX to enable it to meet longitudinal stability requirements for certification.
However, the system is only needed to enhance stability with slats and flaps retracted at very light weights and full aft center of gravity (CG). The aircraft exhibits sufficient natural longitudinal stability in all other parts of the flight envelope without the MCAS to meet the rules. Boeing emphasizes that the MCAS is not an anti-stall or stall-prevention system, as it often has been portrayed in news reports.
So, yeah, MCAS was added to meet the certification
at the extreme edges of the envelope. Was it cheaper than designing an entirely new aircraft? Well, sure.
It's an effective, safe solution to longitudinal stability problems at the extreme edges of the envelope. I would venture to say it would be a one in a billion (or more) situation where an airline launched with the CG full aft. In ANY of their aircraft. The airlines compute weight and balance for every flight.
To construe this negatively as a way to put profit ahead of safety is, again and IMO, the reasoning I'd expect from somebody about 12 years old.
Again, for emphasis, both the Lion Air and Ethiopian crashes were not the result of MCAS. In the case of Lion Air it was a non-airworthy aircraft with an AOA problem. In the case of the Ethiopian aircraft, it was most probably a bird strike causing an AOA problem. The solution was pretty simple, as the OTHER Lion Air crew demonstrated the day before.