I wish it worked like that, but I donct think it does. Connecting clouds means introducing many complex problems. Data synchronization and avoiding split-brain scenarios, a network setup way more complex, stateful storage that needs to take into account all the quirks and peculiarities of all services across all clouds, service accounts and permissions that need to be granted and segregated for all of them, and way more. You may gain resilience in some areas, but you introduce a lot more things that can fail, be misconfigured or compromised.
Plus, a complex setup makes it harder by definition to identify SPOFs, especially considering it’s very likely nobody in the workforce is going to be an expert in all the clouds in use.
To keep using your simile of the disks, a single disk with a backup might be a better solution for many people, considering you otherwise might need a RAID controller that can fail and all the knowledge to handle and manage a RAID array properly, in addition to paying 4 or 5 times the storage. Obviously this is just to make a point, I don’t actually think that RAID 5 vs JBOD introduces comparable complexity compared to what multi-cloud architecture does to single-cloud.