System dependability is a measure of the degree of users' trust in a system. This degree of trust is derived from three inter-dependent characteristics; reliability, availability and security.
Resilience to cyber-attack refers to the ability of a system to resist attacks and perhaps recover fast even when the system is breached.
Looking at the above definitions, it is easy to see that for a system to be trusted by its users, such a system needs to be resilient to cyber-attacks. This is because frequent attacks on a system can result in an unwanted experience for the users of such systems. This is why resilience to cyber-attacks is critical to system dependability.
I will give a good example to demonstrate this. Recall in 2018 when Facebook was breached and accounts of 50 million users were exposed online. If you read up the story, you will learn that several users lost their trust in Facebook because the system was not resilient enough to resist data breach.
Hope you found this answer helpful. Feel free to share this answer with other students in your network.