Websites have no reason to use JavaScript. They should be a collection of documents, possibly generate dynamically on the server side, but to the user HTML+CSS that can be viewed in a browser, downloaded with curl, viewed as source code, etc.
Web applications have no reason to exist. Basically we are turning the browser into a JVM, a way to run code portably. But this have nothing to do with the web! It's just bytecode loaded from a remote server. What Figma, or GMail, or Google Docs has to do with a website? Nothing.
These things shouldn't be implemented in a browser, it's not the right place. These problem should have been solved at the operating system level, but it was too difficult for Microsoft to agree with Apple and the open source world on a portable operating system interface to made possible universal binaries among platforms, with maybe a bytecode, maybe a way to load code from an URL.
No they couldn't agree, so they took the only really open standard, the web, and bloated it, basically making it an operating system (well, Google even made it really one!), what's the point? That now most people run Windows or macOS with on top a browser (probably Chrome) into which they run 95% of the software that they use. Does this make sense? To me is a huge inefficiency, that only makes things slower, slower than they were in the 90s to me (even if we have order of magnitude more powerful hardware using the GMail web interface feels slower than what it was using Outlook on Windows 98).
Put another way: "websites have no reason to execute code". Is a document with an embedded form a website or a webapp? what if one of the inputs is custom and requires some custom validation logic is the expectation from your train of thought that this logic must only execute on the backend?
Websites are interactive documents and they have been since the inception of the hyperlink. I struggle to draw a line where a website becomes a web app outside of the obvious examples at far ends of the spectrum.
Although you probably (?) want to add that OS maintainers are slowly turning their repository of available OS-level applications into walled gardens, which I could see may push some developers to currently prefer web application implementations.
In the same way that writing a REST API is "connecting to some DB and simply exposing data via some endpoints" or building a machine learning model is "simply throwing some data at some algorithm and getting a bunch of analysis out" or devops is "simply starting some remote servers" or creating an OS is "simply writing a bootloader and then some stuff on top."
Remember, my comment was a response to the claim that something like Figma could not be done before the current frontend development complexity mania. But there is nothing in that collection of bad tools that is necessary to write a canvas based UI like that. It could have been done before and it most likely has been done before.
I think people are forgetting how bad it used to be. Loads of jquery spaghetti code everywhere rerendering the page 1000 times with a nested for loop.
Also, web applications have became so much more complex than they were (but still work!). Things like Figma would be unthinkable even a few years ago. And - even though it's running in a browser Figma feels far more responsive than Illustrator or Fireworks (RIP), plus it has a load of collaboration tools which these desktop apps don't have.