Plain text is great for data that is just that - plain text.
As soon as data isn't text, the idea falls apart quickly. What would be the advantage of storing audio data as text? It would take up more space, be ambiguous, and would require more memory and compute time to load and save.
Same applies to many other forms of data that aren't inherently text or supposed to be manipulated manually.
Everything is not text and everything is not intended to be manipulated and interpreted by humans directly. This applies to audio and video data, but also to a lot of specialised data formats that are optimised for the algorithms that use them.
So no, we won't always be returning to text and "leaving" text formats is a natural and important step for data that's not generated manually for and by humans (i.e. by means of manual data entry).