There's nothing wrong with relaxed stability, autopilots are 60 year old technology and have been keeping people safe since our grandparents were flying. And the certification process has for the most part produced very safe aircraft.
But in this case, the certification process was seen as too expensive. Boeing wanted to market a tweaked, high-efficiency 737 with newer engines, and didn't have the time to spend on certifying a whole new aircraft. So it used the same old one and the faster regulatory process.
But that assumes that the regulatory process worked. Clearly that cascade of changes was not in the spirit of "just small modifications to existing designs". But the FAA didn't squawk and certified it anyway.
And as it happened, just one of those choices (using the trim for stability control) turned out to be unsafe in unexpected ways. One was that the AoA sensor driving the thing was glitchy. But more importantly, the trim was originally certified on the assumption that a trim control failure could be overridden by the pilot, but no one realized that pilots weren't trained to think of trim as being a computer controlled thing and didn't understand what was happening when the MCAS started fighting them.