True. As we don't have a counter-factual; we can only speculate about the fate of the marbles, had they been left in place. We do know that the British took very good care of them — so, at least they continue to exist. Now, the only issue is where they should be.
A robber can take very good care of what he steals—doesn’t make it right. Especially in light of the fact that the people who owned them were a subjugated population at the time. Very tacky and despicable on the part of the Brits.
As for taking “good care” of them - not really. The Brits in fact have been the worst of caretakers. Over and over again. Wiki has a suprisingly good overview of this damage too-— see “damage - “British Museum” and “elgin”
http://www.guardian.co.uk/uk/1999/nov/12/helenasmith
http://en.wikipedia.org/wiki/Elgin_Marbles#cite_note-guardian-32