So what has changed?
It's not quite as simple as that -
(a) certification processes are very comprehensive and address a generalised set of requirements held to provide a very high probability of successful in-service operation.
(b) the aircraft as it comes through the certification process will have shown compliance with the set of certification requirements pertinent to the design
(c) however, the certification standards don't provide an absolute guarantee that the aircraft will be able to weather every conceivable set of circumstances with which it might be tested in service
(d) sometimes (and this may take quite some time to materialise) a set of circumstances will arise which conspire to defeat the capabilities of the aircraft and the presumptions which went into the design and certification processes. It may turn out that this sort of situation confronted the crew of the mishap aircraft which is the subject of this thread.
(e) sometimes a comparatively simple problem within the state of the art knowledge combined with significant design advances will present a problem. The early Comet fatigue problems are a useful instance of this.
(f) sometimes the man-machine relationship may not quite work as well as was presumed and intended. This sort of consideration may turn out to be material in the present case under discussion.
It is common to see progressive minor changes to an aircraft Type as in-service knowledge is accumulated.
Sometimes such development is subtle in that it is not evident to the wider Industry. Instances include routine engineering design and maintenance practices changes. All Types experience this sort of development throughout the life of Type.
Sometimes changes may need to be effected quickly and these usually end up as Airworthiness Directives. I can't bring to mind any aircraft which has not been the subject of ADs.
Very occasionally, a Type may have problems arising sufficient to warrant a revisit of the Type Certification process. Not many Types are subject to this re-examination.
At the end of the day, do I worry about flying on either a Boeing or Airbus (or any other reputable) aircraft ? Of course not.
Having been involved in the design and certification processes for just about all of my working life, I am content to accept the low risks inherent in flying in certificated aircraft and equally content to accept that they are not perfect and come with flaws.
If the flaws are minor, such are worked around by engineering, maintenance and operational standard practices - if major, by modifications, as appropriate, to aircraft or procedures. All part of normal aeronautical processes.