When you create a modern webapp, a lot of data are retrieved from servers as Json and formated in the browser in Javascript. Even sometimes Css is generated on browser-side. Even more, on webapp where user login is taken into account, the display is modified accordingly.
That's the web of 2019. The approach consisting of geting remote files and launching them in a browser is really naive.
Speaking of SingleFile, it as a cli version and can handle full web 2.0 webapp without any problem. And of course, the Web 1.0 webapps work as well.
In terms of React at least, fetch requests are not a part of the framework in any way and any present would typically be done in custom code in lifecycle methods. Even Redux, is—by default—client-side only. Stores are in-memory, actions populating them would make fetch requests with React/Redux-independent logic.
Other JS frameworks are, typically, the same. And all of that is just considering dynamic XHR. Loading scripts is much less typical, and never required. The most common application of this I've seen is the GA snippet, which mainly does it to ensure the load is async without relying on developer implementation: it's 100% unnecessary to do it this way.
So yes, unless you're distributing a tracking snippet that you expect non-devs to be blindly pasting into their wordpress panels and still have it work efficiently, generally speaking use of this method is never necessary, and commonly a red flag for poor architecture.
Web 2.0 refers to the use of ajax. This refers to the early 2000s sajax, jquery..
If you want to separate angular, react, vue maybe it's web 3.0.. but wasn't web 3.0 referred to as mobile?
Ajax will* still work fine with an internet connection as long as those ajax endpoints don't require cookies and don't linkrot.
* not 100% sure how the tool handles relative URLs embedded in source : if it's not clever enough though, this is very fixable via PR (as in its not an architectural limitation)
Tbh, often those are superfluous, or egregious examples of bad web dev, so it seems a reasonable solution for most cases.
SingleFile is a different approach, but it's a lot more involved/less convenient than a cli, and loading in something like WebDriver on the cli for this would be overkill, unless you're doing very serious archival work.