Force-deleting half of index directories didn't help making the cluster back alive.
I tried doubling the master node's memory, but it still dies.
My initial plan was to make the cluster back alive and re-indexing everything.
(with my-index-YYYYMM pattern)
Since the cluster doesn't easily get back on, I plan to create new cluster and find a way to restore data from the dead cluster. (Not sure this is possible at the moment.)