The hard part is simplifying complex things. We have so many odd pieces (usually due to legacy reasons or conflicting designs) which don't quite fit into an otherwise simple solution.
Plain text could probably work for most forms of communication and data formats. It's when you need to separate, categorize, and interact with the information that you need more than plain text.
This website briefly addresses plain text's shortcomings, so to add to that, in terms of software UX, I would say plain text should be used as a starting point while adding interactive elements (e.g., buttons) and layouts only as necessary. By layouts I mean visual separation of categorized information via whitespace and/or color variations. I've found that this results in extremely intuitive interfaces that are both easy for users to digest and developers to maintain.
We seem to be following a trend where "winning" software has little to no learning curves, as they are becoming the simplest possible tools for daily use. I'm not sure if it's a result of the fact that most people are tech savvy, or if we're just getting better at building software and designing interfaces. It's probably some combination of both.