Let me preface by saying that I don't have an immediate solution for this case, the details seem to be application specific. However, I will note that JSON is brutally inefficient by almost every computational metric that exists and it therefore is quite expensive unless your problem domain is trivial. I used to be in the position of spending -- literally -- millions of dollars per year on JSON because that was how people wanted to format the data. You can solve many engineering efficiency problems for millions of dollars/euros.

Which is to say I don't have an answer for why streaming JSON isn't valid in this case, but I can also say that if it was up to me I would never us it in an application that mattered. It is much too expensive for many (most?) applications.