I agree. My opinion is that once you've trained yourself to use virtual desktops efficiently, multiple monitors becomes more of a hassle than a benefit.
I think multiple monitors is the solution for people who would rather solve the problem by spending their money instead of the effort it takes to configure and become accustomed to switching between virtual desktops. Given that it is a strict biological limitation that the human brain can only focus attention on one thing at a time, I don't believe there is any valid argument for why moving your eyeballs between physical monitors is any better than hitting a key combo to switch between virtual desktops on a single monitor once those key combos have become muscle memory. Additionally, the number of physical monitors you have is limited by how much money you have to burn and how much physical space you have to place them, whereas virtual desktops are theoretically unlimited.
There are some things that don't need to be actively looked at most of the time, but need to be visible so that you know when something happens that you do need to pay attention. You could do it by polling—put it on a virtual desktop and switch to it every so often—but that adds latency and can be even more distracting than having it visible in the corner of your eye. Think of things like Element or Slack or a dashboard that tracks bugs/issues/alerts.
Then there are reference displays that you look at on demand. Most of the time switching virtual desktops is good enough for this, but not if you're following along with a sequence while actively working.
Then there are things that are just big. Perhaps you're displaying an autogenerated graph, or you're using an information-dense tool (maybe with multiple relevant layers).
Not to mention wanting to consult things while on a video call, which constrains the screen to use based on camera positioning.
I very actively use virtual desktops, yet I have two external monitors in addition to my laptop screen. Most of the time, I really only make use of one of the external monitors, but situations arise that require both. They arise frequently enough that I notice the lack (eg when I'm fighting with my configuration and only one is working, or I've loaned one monitor to someone else). And when I'm mobile and down to just the laptop screen, I definitely notice and even adjust what I'm working on to avoid losing productivity.
I think multiple monitors is the solution for people who would rather solve the problem by spending their money instead of the effort it takes to configure and become accustomed to switching between virtual desktops. Given that it is a strict biological limitation that the human brain can only focus attention on one thing at a time, I don't believe there is any valid argument for why moving your eyeballs between physical monitors is any better than hitting a key combo to switch between virtual desktops on a single monitor once those key combos have become muscle memory. Additionally, the number of physical monitors you have is limited by how much money you have to burn and how much physical space you have to place them, whereas virtual desktops are theoretically unlimited.