While it may be true that many flights are completely full these days, you're statement above is a false premise. The 6 UA flights you took in May out of thousands operated hardly is a reliable sample size. This can also vary based on route, time of day, etc. Just saying.

That said, I always do book the best seat based on the map at the time, while expecting that the flight will become full, and the seat next to me occupied. Then I check every so often to see how things are shaping up. If the seat next to me remains open, I consider this a bonus, not an expectation. When flying with a companion, I now make sure to book seats next to each other. When loads were down a couple of years ago, there was a good chance if you booked an aisle and window, the middle would remain open, but not likely on most flights these days.