However if we want to be accurate, even if I agree with what you mean, NO climb speed, including best angle and max rate are sustainable on our aircrafts...
Oh you cheeky bugger!
Now you're using a different meaning of "sustainable".
I agree with your statement about zoom climbs but they shouldn't be referred to as "best angle" climbs. That's incredibly confusing especially when trying to teach a student what best angle (Vx) climbs are. Discussing the benefits of the zoom climb, like you did, is good but it shouldn't be called Vx (best angle) because that is something different.