To the substantive comment: if your JSON is in a DB, then the DB can do whatever fancy thing it can come up with in order to replicate changes. Databases are good at that, and a place where the engineering effort is well-spent. Both sides of the JSON Patch communication can then read and write to the DB and everything will be fine -- but there'll be no need for JSON Patch, no need for any application-level diff generation and application at all.

As for working at Mozilla: oh, it's worse than that, I'm the one who did the most recent round of optimization to JSON.stringify(). So if you're using Firefox, you're even more vulnerable to my incompetence than you realized.

Furthermore, I'll note that for Mozilla, I do 90% of my work in C++, 10% in Python, and a rounding error in JS and Rust. So although I work on a JS engine, I'm dangerously clueless about real-world JS. I mostly use it for fun, and for side projects (and sometimes those coincide). If you expect JS engine developers to be experts in the proper way to use JS, then you're going to be sorely disappointed.

That said, I'd be interested to hear a counterargument. Your argument so far agreed with me.