[withinboredom]

If a cell dies, oh well. You then run a "rebalance" job to move users off the dead cell. If the cell has downtime, that doesn't apply to your overall downtime. In other words, maybe all central-us customers cannot use your service; but it eventually comes back up. If it is dead-dead (ie, nuke or catastrophic failure), then you would migrate those users to a new cell -- at the expense of latency for those users.