"\n" (that is, ASCII character "\", followed by ASCII character "n", producing escape sequence "\n" for newline) is not prohibited inside a string literal in that model. Only actual newlines (that is, ASCII characters 10 and 13) are prohibited, and that is already a part of JSON spec.
Like I said, the only place where JSON permits actual newlines is as part of insignificant whitespace. But because it's insignificant, newlines can be replaced with any other whitespace character, and the resulting JSON will have the same exact meaning.
And why would grep lose "order of magnitudes in speed"? If you were actually using grep, it'd work exactly the same as it does for any text file. The only thing that JSON does is add some structure to the contents, so that it can be easily converted to some tabular format that's more convenient for structured queries, aggregation etc. But you can still treat it as plain text for all purposes.
Like I said, the only place where JSON permits actual newlines is as part of insignificant whitespace. But because it's insignificant, newlines can be replaced with any other whitespace character, and the resulting JSON will have the same exact meaning.
And why would grep lose "order of magnitudes in speed"? If you were actually using grep, it'd work exactly the same as it does for any text file. The only thing that JSON does is add some structure to the contents, so that it can be easily converted to some tabular format that's more convenient for structured queries, aggregation etc. But you can still treat it as plain text for all purposes.