Anyone can come up with an excuse to justify their actions, doesn't mean it is applicable.
No one was appointed custodians of art to preserve art you didn't create. Anyone who is capable of making it, should get to keep it, whether or not one thinks they can preserve it.
I'm not particularly happy that these items were literally stolen to British Museum, but your line of thinking leads to cultural disasters like when Syria was taken over by terrorists in 2014 and they destroyed all the artifacts (on purpose).
What happened to Syria then was e.g. threatening to happen to Iran in the 70's.