How can To what extent do the 737 MAX problems affect Boeing's other airliners? be improved please so as to be on topic?
I was one of the people who closed your question as opinion based. The box displayed at the top of the page says:
Update the question so it can be answered with facts and citations.
In its current form, the question cannot be answered with facts supported by citations because Boeing will not publicly say that they have a systematic problem affecting multiple aircraft series. Of course information from whistleblowers could be used to support an answer, but the article you quote from does not nearly show enough detail on other models to do that. They only mention the 777X, and comments like
similar shortcomings in the flight-control systems
do not make any sense considering that the flight-control systems of the 737 and 777 are completely different and are not detailed enough for an answer anyway. There simply isn't enough public information available for a fact based answer.
How can the question be improved?
I personally think, it will be very hard to salvage this question. You could maybe restrict yourself to the technical aspects only, like e.g. Is MCAS used in other Boeing aircraft? or Are other 737 variants affected by the same problems as the MAX? (we may already have a question like this). Any question about problems with Boeing in general or about the upcoming 777X will be hard to answer in a fact-based way.