You can download content from Disney plus and view it offline, but you still need WiFi or wired network connection to be able to cast it to Chromecast device.
Casting betwin two devices requires a network.
Alternatively, you can upgrade to Google Chromecast with Google TV and expand its storage capacity using a powered USB-C hub and an external storage device (thumb drive or HDD). This will allow you to download Disney content on Chromecast with Google TV internal storage and view it "offline", although you still need to be online to be able to watch its applications content.
Why pay money for a streaming service?
Well, because it offers you content you want or exclusive content, not available on other official and legit sources, and you have to pay a fee for the right to watch it, not own it. If you want to own the content you can buy that release on a physical format, like DVD or Blu-ray, and you can watch it whenever and wherever you want offline.
