If the DC legal team knew their TM was indefensible, they ought to have picked their battles FAR more carefully. Because sending a demand letter to a target likely to contest the TM, would be the end of the TM. Which is exactly what happened.
They could have literally kept their TM - to sue another day - if they had chosen to look the other way with superbabies.
So, my point was whether it was hubris that led to the decision to go after superbabies, or failure instituting unsexy (but necessary) internal dilligence checklists.
A trademark is not infringed if its infringement is not known to others accused of the same thing.
One special aspect of trademark law is that you can also lose a trademark by failing to protect it.
Letting one more usage slide would not make "Super Heroes" more generic.