Photo Stream only holds photos on the server for "1000 image / 30 day". What is on your device is on your device. If you were to sign out of iCloud then back in you would only get "1000 image / 30 day".
iCloud is more of a sync toll that backup. You should be backing up your photos on your computer. You can have Photo Stream sync to your computer.
If you have old photos in your photo stream on your mac, be sure to move them into a permanent album, otherwise if you some day turn off PS or have to reset the device and restore, the stream will be be set and you'll only find the latest 30 days' worth of photos in the stream album - older ones will have been deleted. If those were moved into another album, then they will remain.