They can design a solid system, although never perfect. In this case, sadly, they failed to design a solid system. That is not in question. Even Boeing have effectively admitted this.
The question now is, “how and why did this happen?”, especially in such a tightly regulated, conservative and safety-conscious industry, with one of the world’s most respected organisations. The answer to that is now under extensive inquiry.
We may or may not ever discover the truth, but typically when this many people die unnecessarily, the difficult questions finally get asked, and those with the answers are forced to actually answer.
Of course, it will fundamentally point to one thing: MONEY.
But we aren’t going to get rid of money, so it’s the movement of money, to whom and how much, and how much those in charge of money get to influence those in charge of solid design, that will come into question.
Well, if by operator, you mean the airline, well they are in charge of employing sufficiently skilled pilots and providing them sufficient training to get their passengers from A to B safely. The airline is also in charge of purchasing sufficiently safe equipment to do so, and maintain it to a sufficient standard.
If they fail to do this, they must accept some liability for the failure. Whether they failed to do this is not yet ascertained - some think the pilots should have saved the passengers from the design fault, some think that’s an unreasonable ask, and some think it was impossible.
If, on the other hand, by “operator” you mean the particular pilots involved, well, in my opinion, pilots have a personal obligation to maintain a standard on top of that required by their employer and their regulator. They can also be held personally liable for the failure, and depending on the global jurisdiction may be supported and/or indemnified by their employer, or, not.
Aviation safety is a rich tapestry, that normally results in a high level of safety, but many argue that it is gradually being eroded in standards of design, training, and skills, generally by one thing: MONEY.
Think about that next time you buy the cheapest ticket available, because that MONEY only comes from one place: you. The less you pay, the less MONEY available to pay someone to design a solid system, or to pay or train the pilot to save you from the less-than-solid system.