better security for the entire world...

The moment Linux takes over as a dominant desktop/laptop OS we'll start seeing a metric ton of the windows hackers follow suit to attack us. We'll end up in a situation where they'll probably go after some random kernel bugs that nobody else.has found yet or just don't think are critical/exploitable. Or they'll just attack the biggest, most widely used distros, going after people using them and any derivative distro similar enough for their malicious tools to work on it.

In general though, it would be a good thing for Linux to become a lot more prominent in the desktop/laptop market for general users. Especially since I imagine thanks to Linux being open source, people would be able to stop these malicious actors from doing damage much quicker (even though I imagine the majority of normal people switching over would almost never update because they're used to forced updates and not having to do it themselves).

There will be more diversity in software and distros

I wish, but I doubt it. If we get to the point where there is a mass migration from Windows to Linux, it will almost certainly be concentrated into one or maybe two big distros. Probably Ubuntu.

Today, most proprietary software vendors only support Ubuntu and RHEL. Look at AMD. The ROCm installer supports Ubuntu 22.04, RHEL 9, and SLES. That's it. Not even modern versions of Ubuntu. And it's extremely ornery about dependencies. Python 3.8 or 3.10 required! No 3.9! No 3.11! Trying to get it to install on any modern Debian-based distro is the ninth circle of Dependency Hell.

Right now it’s sort of up to Nvidia and Wayland. Desktop sound is in good shape, desktop color (profiles and matching) and fonts are not there yet. Ray tracing and hdr have proven how much of a second class citizen desktop Linux is, so right now the most important factor is the SteamDeck for pushing the envelope to implement new tech. Chinese and German goverments moving to Linux helps but to be honet, I think that the “office and browser” use case is pretty well covered.

Possibly. But it's also pretty common in many instances of technology adoption that as more users come, the quality gets worse, and while open source doesn't have to worry about a shareholder-driven profit motive driving it, it's still easy to wind up with a muddled focus. I wouldn't expect that Linux and all of the associated software projects that make the functional desktop are going to be an exception overall. If you're an open source developer working on a project now, basically any user is some form of power user, and it's easier to find consensus of what to prioritize on a project not only because Linux users tend to be better about understanding how their software works and are actually helpful in further development, they're also likely to direct development towards features that make software more open, compatible, and useful.

Now fast forward to a future where Linux is the majority desktop OS, those power users are maybe 5% of the software's user base, and every major project's forum is inundated with thousands of users screaming about how hard the software is to use and, when bug reports and feature requests are actually coherent, they mostly boil down to demands for simpler, easier to understand UIs. I can easily imagine the noise alone could lead to an exodus of frustrated developers.

Some things are better for NOT trying to be the answer for everyone.