Isn't the concern of downtime and hacking only relevant if you have reason to believe that they are more likely to happen with the CDN than with your own servers?
If you host some stuff on server 1 and some stuff on server 2, and you need both to function, then you have two points of failures.
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This is kind of a simple argument, increasing the number of "points of failure" doesn't, or shouldn't increase your odds of "failing" more. Adding cache layers and CDNs may add "servers" to your architecture but should also be done in way to reduce overall downtime.
Increasing the number of points of failure absolutely does increase the chances of failure, even if each 'point' is more reliable. 1 server with a 98% uptime is more reliable than 5 servers with a 99% uptime, if all 5 have to be working for everything to 'work'.
