Everything in engineering is a trade off. The changes that were made were not inherently dangerous, but the certification process on those changes fell apart for a variety of reasons. If the certification process had been followed correctly, these planes would be flying safely.

The FAA is underfunded. A pro-business administration applied pressure to help Boeing certify more rapidly. Due to time and money pressures, many things that should have been performed by the FAA safety inspectors were actually performed by Boeing engineers. The safety assessment was performed on incorrect numbers! (Boeing estimated some parameters and performed safety assessment. Then those parameters were updated after flight testing, but the safety assessment was not re-performed with the updated numbers! That's a huge problem in the process.)