That assumes the prevailing theory of malicious pilot was correct. Indeed if he did manage to turn it off early it might have contained something.

However if that theory is indeed true, it's clear that he wanted to disappear without a trace. In that case it would have made sense to keep it running especially because it only keeps the last 2 hours.

But yeah if it had been longer he would have turned it off in that scenario. It would be best if the CVR had a backup battery (and internal protection is that shorting out). In fact I remember reading in several admiral Cloudberg articles that the CVR and CDR data was incomplete due to bus power loss during accidents.