You spend a week building a castle with legos, and suddenly your mom asks if you can change some parts to use the new <lego competitor>. You can try to make the old and new parts fit together, but it isn’t going to be easy most of the time, and you can’t be certain that the lego competitor will have the same pieces or do the same things as your lego version. By the time you are done redoing those parts, you’ll end up having to recreate large portions of your castle to make everything work together again, and even then you might miss something important that breaks a functionality of your castle.

Maybe in this case "ELI5" stands for "Explain it like I have 5 yrs experience in software development but gave up in year two"?

ELI40YO Engineer

Arguably, if the five year old was reading hacker news, they might. :) Point taken, though, but honestly this doesn't seem like the place to simplify things to quite to 5Yo level.