You clearly have absolutely no background in aerospace engineering. The 737 airframe is actually relatively stable compared to many other things that fly ... for instance, many high-performance jets will literally begin to oscillate and tear themselves apart without help from their flight computers. The flight computer is a part of the airframe, and it is perfectly valid for the flight computer to contribute to the airframe's handling characteristics. Boeing's mistake was in underestimating the burden placed on the pilots in a runaway trim situation, and assuming that pilots could execute a recovery procedure correctly that remained unchanged from prior iterations but that had also been rarely needed on prior iterations (vs. the MAX, where MCAS failures were common enough that an average pilot might actually need to execute the procedure).