I used Mac since the 1980s, and been able to tile windows, stack windows, and overlap windows. The Mac would remember size and placement. IIRC, the Mac II could support windows spanning across up to nine monitors of various resolutions, shapes, and bit-depths. MPW had window splitting which was annoying. The Finder has/had Zoom which would resize the window to the smallest size with no scroll bars. I've wished there was an easy way to implement Zoom in other applications.
With small screen phones, the screen is usually filled with only one app/mode at a time. I see a lot of the same use on Windows where there is only one maximized app/mode at a time, even though one can tile, stack and overlap multiple windows. Window's Multiple Document Interface (MDI) has somewhat morphed into "Tabs" which still present only one tab/mode at a time. No modes please. I am looking at the screenshots and videos for i3 and see a lot of scrollbars and wasted screen space. I don't see the advantage of i3 over Mac or Windows, or XFCE that I use in Linux. What am I missing? Maybe I'm good with what I've got.
It's easy to come across lots of overly done up i3 setups, often using "i3-gaps" which is a modified (fork, I think) of i3 which introduces gaps between all windows and the screen edge in the name of prettiness. This is common in the /r/UnixPorn community and frankly, I would never use it. Perhaps you're seeing a small subset of potential i3 setups. On scrollbars, I don't feel like I see any more scrollbars than in Mac or Windows. Probably less?
The main benefit I see in i3wm compared to any other window managers in other OSes is actually the use of space, my windows are literally edge to edge. Apps don't have title bars unless you've got your windows stacked, and you can customise exactly how many pixels the title bars are. You can customise the colours of active, hovered and inactive window titles. You can add borders to windows, of any size and colour, you can have none.
I also love the multi desktop feature which I use prolifically. I have 10 desktops named 1-10 but you can call them anything you want (emojis work too, thanks unicode). Certain apps always open in certain windows so I always know where everything is. This is all declared in a config file so it's never a guessing game - it just works. Also, I have 2 monitors and multi-monitor support for window management works a dream.
It's not just the tiling - moving windows around is so easy, I can press $mod+Shift+5 to move a window to desktop 5, or $mod+Shift+Left to move it to the left, or even $mod+Shift+Alt+Left to move it to the left monitor. These are second nature to me now.
Finally... the speed. Wow, switching between desktops, windows, monitors just happens incredibly fast. When I use a Mac or Windows I just can't stand all the animation and slowness. M1 macs included. This is in part helped by the picom compositor, with animation time reduced to being nearly negligible whilst being enough to "look nice".
Those explanations make more sense. This feels like it is more about personal preference. I have three monitors, one of them is 4K.
Probably the same problem
Scroll bars are due to the layout of content within the windows. Unfortunately, I need to use very poorly laid out apps/windows that need to span across the monitors! The design where there is a tree on the left, but one can only select one node/mode at a time, and a usually minuscule amount of content on the right appears within an enormous pane of wasted space just grates on me. There are betters ways to layout content minimally, but like the Zoom feature it is a lot of effort.
Screen real estate
Window snapping edges down to the pixel (0 pixel gap.) Lovely. I can't stand window snapping and turn it off completely. I want title bars. I am not fussy down to the pixel where a window lands when I move or resize it. Since I depend on overlapping windows, I want to see that hidden window between my irregularly spaced gaps between windows.
Keyboard-driven workflow
My workflow is mouse-driven. I prefer only a few well-known short-cut keys as the author described in the article. My left hand hovers over the Command keys, and my right hand is on the mouse. In Terminal, I have Autokey remap all the legacy ctrl keys — I can Copy Ctrl+C and Paste Ctrl+V in the Terminal consistent with other apps I use.
setxkbmap -option "ctrl:swap_lalt_lctl"
Flexibility
config files! I would much prefer if, like decades ago, the system would automatically save and restore window placement.
Workspaces
By "the multi desktop feature" you mean Workspaces. I turn Workspaces off completely.
Speed is relative. I don't have an M1 mac. I still have some working PowerPC Macs and the speed is fine. On the i7 Dell, I turn off animations and compositors.
With small screen phones, the screen is usually filled with only one app/mode at a time. I see a lot of the same use on Windows where there is only one maximized app/mode at a time, even though one can tile, stack and overlap multiple windows. Window's Multiple Document Interface (MDI) has somewhat morphed into "Tabs" which still present only one tab/mode at a time. No modes please. I am looking at the screenshots and videos for i3 and see a lot of scrollbars and wasted screen space. I don't see the advantage of i3 over Mac or Windows, or XFCE that I use in Linux. What am I missing? Maybe I'm good with what I've got.