I would ageee with you for cases where both sides had a strong argument, but in Snapchat’s case everyone was complaining about the new UI, so I’m not sure how to describe that sense - people who complained are from all different ages & backgrounds so it’s not like they had UI design experience or anything - “common sense” is probably the only thing they all had in common and that thing alone was enough for them to determine the UI sucked.

In this case, simply doing some basic user-testing would’ve been considered common sense and would’ve brought up the problem immediately.

In which case, they didn't need "someone with common sense to take a look at the new design". They needed basic user testing.

Telling a company "do some basic user testing" is a useful recommendation. Telling them to have "common sense" isn't.

Basic user testing would’ve been recommended by anyone with even a tiny bit of common sense.

Either the company truly didn’t have anyone do user testing, or they did and were ignored (which doesn’t surprise me either considering the child they appointed as “CEO”).

This is tautological. If you would like to rant, just rant. Talking about some mystical quality that makes things turn out well is circular.

I'm not sure why the quality I'm talking about is so "mystical", when hundreds of thousands of people on social media seem to have it just fine (given their reaction to the redesign), as well as pretty much anyone who talks about the Snapchat UI.

Maybe the problem isn't with my rant, but that somehow the people running that company do indeed not know what they're doing?

I'm afraid you keep missing my point. If you have some specific question about it, feel free to ask it. But I don't feel like me giving another explanation will move things along, so if you're content not to understand it, that's fine by me.