If you didn't explain why X doesn't work, it's absolutely justified that people ask if that simpler solution X would work as well. Assuming you considered "the obvious answer" is unfortunately often not sufficient, as what's obvious to one person might not be obvious to another person.
The "just" questions have their place, but more often than not, I've found it there's more than one they use up the available conversation energy.
By the time all the obvious "justs" have been asked, and explanations why not for the n'th time have been given, the worthwhile meeting energy is exhausted and the original purpose of making a proposal is rendered pointless. You can see this outcome long before the end of a meeting too. If you see it coming at the start, maybe you won't bother making the proposal in the first place.
It's not a bad sort of question, rationally. It's more that it tends to be accompanied with dismissive, looking-down yet shallow thinking, when what you needed for a useful meeting was engagement with the "meat" of the issue, not shallow dismissals.
I knew someone who worked at a place where the word "just" in conversations was called out humorously with a silly noise, even when said by accident, to discourage it :)
