It may not be useful to you, but as someone who maintains a dual-boot Windows/Linux system, I currently have to make an upfront decision about how to allocate storage between Windows and Linux. Having everything in one partition would free me from having to either over-allocate OS-specific space, or risk having to resize partitions later if I run out. It would also eliminate the need to have duplicate copies of certain data across both operating systems.

But the OS partition being shared with Windows isn't buying you these advantages. That is, you can still do all this with installing your Linux-based OS into a fixed size, well-known in advance size (40GiB).

The things that grow over time aren't part of the OS, they are user data, which can be shared without doing anything special -- just mount it up.

> The things that grow over time aren't part of the OS, they are user data

Unfortunately, that's not entirely true, at least for Windows. OS updates and other files are regularly stored in folders that aren't always trivial to put into a different partition. Not that this is enough of a reason to make this abomination your main install, but...

Windows did have an issue where it did not clean the update directory. They've fixed it. You can put your home on another partition, or if not for Windows, you can do it most easily for Linux.

There's solutions to this that are not the OP and are probably easier, people just write them off because "ew CLI."

I'm not afraid of having to do stuff in the CLI. And of course there are a ton of ways to solve it. None of them would be as straightforward or as elegant as having one partition that just contains all my user data and both of the operating systems I need to boot.