I *hate* "why not just" as a reply.

"Why not just do it this other way instead?"

It implies that the other way is faster, easier, or otherwise better, without actually providing any evidence to that effect.

The person who made the original proposal has to research what those advantages might be and see if they actually apply to the current situation. This is information that the replier supposedly already has.

If you know it's better, say why.