Not even a Banksy lasts forever.
Hamburg’s and Germany’s last and only Banksy “bomb hugger” has been vandalized. Strangers sprayed an underlined “grafitti” (sic!) whose paint ran under the 2011 mounted acrylic glass plate. The police states the painting’s condition as “severely damaged”.
Measures to restore and save the painting are in discussion.
This is street art in a nut shell. Crossing is a component of this special art form that is intended to be temporary and nondurable. I don’t think that anybody, who thinks that graffiti needs to be protected by a plate, ever understood what street art really is about. And to be honest: I would not even be surprised if Banksy himself decided to cross his work.