If you host everything on your own server, you have one point of failure.
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.
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.
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.