Of course, it's rather hard to say for sure, and the counter-argument will be "but we could have created that anyway". But just off the top of my head..
- bootable distros. I remember the days when if you wanted to install a new version, you had to just do that. Everyone was thinking "just make the installer easier" but then the bootable distros came along and changed the game.
- Raspberry Pi. If there was only one distro, nobody might think it was worth it to create a small set of hardware if the thinking was only for desktop
- Android? Perhaps. With one distro, there would undoubtedly have been desire for control. That leads to stifling innovation, and just DOING new things.
- the various package managers, which absolutely improved the entirety of linux distros, ability to install, upgrade, and maintain systems.
- DEs. I love XFCE. There are others that are loved by many. Recently there is Cinnamon and MATE. If I had to use some "committee approved" DE I am sure there are things about it I wouldn't like.
- ANY package or app that you like. Maybe GIMP wouldn't be where it is now. Maybe not Libreoffice, or Firefox, or Chrome, or Chromebooks. Because if something didn't get approved by "THE" distro, it would be pointless to make it. Choice is the key, without that as a foundation, it's all about the committee approval.
- Google builds a lot of what it needs in-house, including their own distro. Would any of those variants be "allowed" in a controlled, 1-distro world?
That's the thing... by being an open landscape (you use the term fragmented) things are possible. Things are allowed to happen because no one entity controls it. It's evolutionary. If Debian didn't happen, then Ubuntu didn't happen, then Mint didn't happen. Probably the easiest way to conceptualize it is to look at what happens if it is NOT what Linux is.... Microsoft, Apple. Closed, walled gardens. Failed software and even companies along the way that got squashed because they weren't "preferred" by the powers that be.