Love this. My wife and I are in a similar dynamic. We've certainly evolved over the years. When we were a bit younger, playmates would come and go. Now it's pretty much just her long-term Lover.
We all know each others' boundaries, expectations, and quirks. He's good to her and they have great sexual and plutonic chemistry. I'm not afraid of feelings getting in the way (he doesn't make her laugh like I do). And as we like to joke: While he is amply equipped to handle her in bed, only I'm equipped to handle her feistiness day-to-day.