Tabs are just poor window management.
Even on a mobile form factor where fullscreen makes sense, tabs don't.
The idea of a 1:1 correspondence between apps, windows, and processes is misguided. Why shouldn't one app have multiple windows?
Web browsers in particular are glorified document viewers with delusions of being operating systems (although they are more like EMACS than vim nowadays, with the Lisp interpreter and web apps, so there is that), so why shouldn't each page be its own window?
Because of Apple, of course. Because a computer should be as simple to use as a toaster, with one button that does everything. ("Computer, create a recursive algorithm.")
Because that way you can see at a glance if an app is running or not, and close it. (All of its instances at once.)
The Android workflow doesn't even have the closing of an app. It only has going back to the previous activity. But all documents opened with one viewer are in the same window, whether its the PDF viewer, the image viewer, or the HTML viewer. Each app has its one window with all the opened documents. Even though there is no five-finger close gesture.
On the desktop you usually have a bar listing all the windows into documents already, which works the same as a tab bar does. (Chrome has a separate process per tab, which makes sense. Firefox has one process for all its windows, which only makes sense in MS Windows.)
What could be useful is grouping those windows hierarchically. I have yet to see a window manager do that, but there are "extensions" for web browsers that do that. Using tabs does allow grouping pages in windows (and virtual screens allow grouping of windows), which is a rather shallow tree model. Some WMs, like awesomeWM or i3, are more flexible (but you have to make your groups manually).
I won't be surprised when there will be a web app (i.e. "AJAX", probably written in React) for web browsing that implements its own window management.
"Declutter" was also a feature in MS Windows, for unused desktop icons. In effect, it meant that users periodically recreated the same icons over and over again.