You did nothing wrong. It's first come first serve. You meet all kinds of passengers from the infrequent travelers to the crazy ones. No need to be concerned until an actual issue occurs.
If anything, UA (and more airlines) routinely scare passengers into worrying about getting bag space which makes then line up super early, rush to get on the plane, then panic and stow their back in the first row when there is actually plenty of space in the back. Although some planes seem incredibly inefficient (ie 777), I find most people (and FAs) are very lazy and make zero effort to arrange bags in a manner for maximum storage. I've even been yelled at by an FA for trying to help him find a space for a bag. My mistake, best to just take no action until forced.