Folks are going to justify in their minds that their tang(s) are happy in whatever size tank they have them in. You're not going to change their minds.
Tangs are open water fish and need the room to swim.
Here are two mini stories of my experiences that helped me realize:
1. A local reefer had a 12 foot tank, and I sat there in amazement watching the tangs zoom back and forth at full speed, and thought to myself, "there's a happy tang." Once you see tangs in open water (long tanks), you can appreciate their need and enjoyment to swim with speed.
2. I was involved in knocking down a 450 gallon tank that had a full grown sailfin (enormous) and three of the largest yellow tangs I ever saw. As we prepared to pull out the fish, I commented to the others there that that tank is too small for that sailfin....and we all agreed. The tank just looked small in comparison to the size of the fish.