Cloud files only at the cloud?

I don't believe it is possible to have files Only On the Adobe Cloud servers. Even if you logon to the Creative Cloud website and Drag and Drop files there from any folder on you drive and also have the Sync feature enabled they get RE=Downloaded to the CC Sync folder on your drive.

Sure if you turn of the sync option, actually Pause it as you can't turn it off, but then the only copy of your files is in the Clouds.

Also IF you cancel your subscription all those files will be deleted.

No. Your Creative Cloud storage is not a replacement for hard disk space.  It's an auxiliary space for syncing to other devices and sharing assets with colleagues.   Also do not confuse Creative Cloud storage with dedicated cloud backup & disaster recovery systems like Backblaze, Carbonite and others.  They are completely different things.  See Creative Cloud Storage help below.

https://helpx.adobe.com/creative-cloud/kb/file-storage-quota.html

Not a good idea... I have read messages from people who have had a program problem, and their cloud files have disappeared

You should always have files in at least 2 place... an external drive works if your internal drive is full

A possibility is this USB3 dock with fan to swap 'bare' drives for backups http://www.amazon.com/StarTech-SuperSpeed-Docking-Station-Cooling/dp/B0055PL2YI or http://www.newegg.com/Product/Product.aspx?Item=N82E16820147511