People who need MS Office because once you have to collaborate with others Open/Libre/OnlyOffice won’t cut it;

The average user doesn't need specifically MS Office. But if they do, they do.

password manager via flatpak only

Use (always) your package manager. The trend of using Flatpak has severe downsides as you pointed out.

Virtualbox [...] GNOME Boxes

Use libvirt and the virt-manager UI

Adobe apps won’t run properly

Might as well be the case. I haven't tried.

Gamers because of the reasons above plus a flat 5-15% performance hit

My experience is the other way around.

old software / games because not even those will run properly on Wine

Old games don't run on a recent Windows either. I've tried.

electrical engineers as typical toolsets

If you need specific proprietary tools, you might need Windows or Wine. Depends on the specific use-case. But the 'average user' we're talking about isn't an electrical engineer. If you're a student, try KiCad it's not Eagle but it is something.

specialized hardware

You need specialized software along with the specialized hardware. Again, more niche than 'average user'.

AutoCAD isn’t available

Same. If you need special software, you need special software. It's arguable if the 'average user' needs exactly that. Special needs might render Linux unusable in your situation.

Finding a properly working FTP/SFTP/FTPS desktop client

My file manager does this. And it's more like the windows people do their webdev. I rarely work like this. I don't have a need for WinSCP on my desktop but webdev works fine.

Why do most people use Linux instead of Windows to host their servers, then? Why is almost all of the web powered by Linux if Windows is better? All the devs and sysops wrong? AWS? Almost all cloud services?

Software runs fine, all vendors support whatever you’re trying to do and you’ll be productive from day zero.

Really? I need to throw away printers because people update their Windows and the printer has no drivers available for the new Windows version. Printers stop after a service pack got rolled out and need fixing. People have Ransomware sent to them. Graphics drivers and sound drivers sometimes do silly stuff and don't detect the headphone plug properly. HDMI doesn't switch over to the projector. All sorts of small annoyances and they happen regularly.

It all comes down to a question of how much time (days? months?) you want spend fixing things on Linux

Agree. If you learned Windows and have no idea of Linux, you'd have to learn this now. It takes time. If you had learned Linux, you'd know where the logfiles are and you'd struggle with Windows. Sometimes learning new things (properly) is a good things. Sometimes you can't be bothered or lack time to do it.

TL:DR; the Linux experience might be great but it isn’t for everyone and anyone. If you need to do your job without small annoyances that will curb your productivity it isn’t, most likely, for you.

The 'average user' doesn't need all the specific tools in exactly that version. The average user needs an office suite and a browser, not Eagle and Adobe. If you live in one ecosystem and have to share stuff with your colleagues, you live in that ecosystem. I agree. I have far less issues with my linux machines and debugging is so much easier with them than the Windows machines and servers I had. It's sometimes been days of trial and error to tackle problems there while Linux usually has good debug messages available instead of 'Error 33492, program closed.'

The average user needs a stable and user-respecting system that get's out of the way. They need Office, a browser, E-Mail, a network-share and a working printer. All the specific tools and WinSCPs and so on are additional knowledge you learned during your times with Windows, while the average user struggles with their Antivirus. I agree, it's more complicated for you if you have 10+ years of windows experience and now try to apply it 1:1 to Linux. It doesn't work this way.

(My general advice is: If you want it 100% like Windows: Use Windows.)