As I am myself a COS, I'd qualify your statement. The armrests do go down for me, and of course I always try to fly in Biz or FC. As in life, nothing is 100%, though.

On domestic flights, I usually fly FC, or try to book flights where I will be upgraded.

If I'm in coach, I do book an aisle, so if there is any leaning into the aisle, I can do it. I recall early in the year, when using a DL intl award, I show up, and they had moved me into a middle seat. I had booked an aisle seat months before. Luckily, I always look at my seat when I get my boarding pass, and try to check with the GA.