I think she brain knows it but feels bored and looks bored.You solved it all.
Your wife is not a real good follower. Not even close.
Otherwise she would know that it takes, by the construction of the dance, much much much less time for the woman to be ready to go to the milonga. The world-wide known and respected teacher Homer Ladas claims he can turn any beginner into a ready-for-milonga follower in half an hour. Even if you were taking twice the number of classes that your wife takes, she would still be far ahead from you. Next time she gives you the bored look, tell her to stop it, and to go to an advanced follower who will teach her one thing or two about the learning curves.
You're also right on another point, yes after some time the women are overtaken by the guys. There will come a time when you'll feel that you're giving her a charity dance when dancing with her.
And finally there is a say
"Dancing tango with one's husband is like reading yesterday's newspaper"
This transgression thing, yunno.
However skilled you may be as a leader, she will always find the other leaders more interesting.
Good idea about being old news and less interesting I did not think that but I can see that. Maybe I should do just walking until the last Tanda and they go full Tangomanic the last song of the night. Ha ha funny frown means start walking still frowning means getting off beat ha ha.