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.
We're following a trend where "winning" software has little to no learning curve, as they are becoming the dumbest possible tools, with ceiling placed so low that no learning is happening at all - in first 5 minutes you get to see all that there is to see. It's the result of UX trends driven by SaaS businesses, where the first impression is what gets the sale, making development concentrated on looks over function.
Smaller plain text files + folders are great for separating, categorizing, and interacting with information. Got another file type relevant to that note? throw it in a folder with a matching filename.txt describing it all. Have aliases or shortcuts to make these appear in any configuration you want all over your OS.
MacOS also lets you put tags on any file leading to further customization. Throw in Calendar into the mix for reminders (not proprietary Reminders) mail for communication and sending yourself notes when you don't have your laptop, and MacOS has all the features baked into the OS that most notetaking apps drip feed through a poorly optimized app on a subscription and with proprietary formats.
This system works on any computer built in the last 40 years and will probably work on all computers built in the next 40. Lets see a company like evernote last for that long.
I'm not sure if it's a result of the fact that most people are tech savvy
Most people aren't tech savvy. Again, maybe you are extrapolating inappropriately from your personal experiences, the people you hang out with, etc?
Older generations definitely struggle using software though, so I think they may be a good metric for how simple and easy to use software can be.
Anecdotal experiences could certainly play a role in my opinion here, but I do think I've recognized a pattern over the past couple of decades where winning software interfaces have made a transition from being overly designed and complex to much simpler designs with (usually) predictable interactive element positions and flows.
Think of what software was like when there were "shiny" buttons and small text everywhere with confusing visual flow, and then think about what it is now, usually composed of flat buttons (e.g., the "material-ui" standard) and easy to digest visual flow with predictable interactions. This transition is likely a natural result of figuring out over time what works best and what doesn't.
By tech savvy people, I mean the end users who interact with software on a regular basis. These people (i.e., most people in the modern world, a number steadily increasing) will naturally gravitate towards the easy-to-use, predictable design patterns to which they've become accustomed.
On a related note, I absolutely hate how most mobile and many web and desktop-based apps nowadays don’t have tooltips, so I can’t figure out what some inscrutable icon/button is supposed to do by hovering my mouse on it or long pressing my finger on it. This used to be table stakes.
I use the basic text editor that comes with the OS or Ghostwriter  when I really want no distractions. It's a great, minimal and distraction free editor I discovered recently when I switched to Linux (it does exist on Windows too and there's a macOS beta).
I put it on full screen with dark theme and focus mode on (it highlights the current line and fades everything else away). When I want to go gung-ho I also enable Hemingway mode which disables the backspace key. Love it.
Is there any software that you think stands out as an example of this approach?
In particular, Org leads to very nice and simple text-mode workflows and mini applications. Besides, apparently Amazon used a custom Emacs application (not Org-based) in the early days to do customer service .
I actually very much prefer it to Unix ncurses applications, which tend to be little information silos. In contrast, Emacs makes it really easy to connect everything together thanks to great Elisp APIs. It feels like a really cohesive environment. Unix shines for non-interactive workflows.
I like doing my computing in 3 (actually 4 platforms): Emacs, a browser (Firefox), a terminal (XTerm) and F-Droid. That, plus a nice manual tiling WM (StumpWM) and functional plumbing (NixOS) is a great simple setup.
For example, a good keyboard, no compositor and XTerm reduces latency a lot vs more complex desktop environments. It's really noticeable.
Elisp APIs and your standard editing commands (which are of course the pieces of Elisp that happen to be bound to keypresses). The difference between this and ncurses/other TUIs is profound - as you said, typical TUIs are data silos, just painting characters on the screen. Whereas in Emacs, all the text on the screen is accessible for search, navigation, copying, and even modification.
Even saying "Elisp APIs" doesn't do it justice; Emacs follows the best traditions of Lisp Machines - you can inspect and modify software at runtime, and make it do what you want it to do, whether it wants it or not. If an Emacs mode (i.e. essentially an Emacs app) doesn't provide some functionality, you can add it either way. Want to use pretty third-party autocomplete like Helm for e-mail addresses in To: field in your Emacs mail client, and want it to pull suggestions from some arbitrary .org file you use as an address book? No problem at all. Few lines of code and it just works.
The downside is security. You need to trust everything you run, because as you say it's so open and inter-winded.
The real problem is dealing with SEO spam, not occasional false positives with hyperlink detection.
I mean that's easy to say, the obvious question then is why it's apparently too hard for Google, since they don't do it despite the fact that it would obviously be of value given their algorithm.