There's nothing to say that a Gemini browser (for example) can't choose to display gemtext links to other resources inline; images are a natural choice, but imagine a browser displaying CSV files as nice inline tables with standard affordances like sorting and line highlighting. No special browser support? No problem; lots of tools can work with CSV, and it's not completely unusable when displayed as raw text.
Some things don't currently have great options. We desperately need a standardized vector image format that doesn't have the cancerous complexity of SVG or EPS. A rich, declarative format for describing forms and their validation could be handy. Perhaps a simple self-contained format for interactive visualizations and games, like a stripped-down equivalent of Flash? (I have some ideas there.) As long as new formats are designed to degrade as gracefully as possible without special support and the means of composing formats is flexible enough, an ecosystem as a whole can grow and evolve while each component remains simple enough for one person to understand.
The present web and our universe of broadly supported technologies is very close to this ideal, if developers had a bit more restraint.