The regulator is to blame if certification is so expensive that old tech is left around to kill people.
The 'certification cost' is just the threshold at which the FAA is fine with all the liability being offloaded on to them, for engines made and maintained to spec at least.
i.e. For this cost to actually decrease would require an entirely different society, one that doesn't settle things through the judicial/administrative system whatsoever. But then costs may increase somewhere else.