In the troop section you have the warning that troops used to train will be retired permanently.
I think the same warning would be appropriate in the hero training box.
When I was a new player I fed Bane to a 1 or 2* hero. I was asked if I really wanted to do this but the consequences were not clear. I think fewer people would make the same mistake I did if the admonition of permanent loss were statedas clearly as it is in the troop section.
Thanks for your consideration.