On what scope?
What's thin? The whole thing, or just the apps?
Where is the data?
Are we only concerned with local execution, or do we also have to consider the security of transmissions?
I mean, if local execution is all we're concerned about, then obviously thin computing is GREAT... but the point about the size of the target is true. There is no guarantee that the widespread deployment of thin computing won't simply shift the malicious efforts to focus on the servers instead. And then the harm caused by a single failure will be so much greater that it might even out the increased rarity of failures.