It's much more useful for greenfield software. Mad props to whoever wrote the gcc patch just to get something like this work some of the time, but rebuilding existing software is probably a fool's errand. There's a good reason you see things like the rpm package format include not only kernel but distro. Running on "Linux," BSD, Windows, or MacOS is one thing, but Linux is a bunch of different OSes, not one. A whole lot of software out there relies upon assuming you're using a particular DNS provider, filesystem hierarchy that may or may not be FHS-compliant. When I went down a rabbit hole a few years back trying to extend Linux from Scratch to include a bunch more packages, I found that GHC won't build if the file "/etc/os-release" doesn't exist. The file can be empty, but it has to exist.

Someone mentioned busybox elsewhere, which sure, will build with this if you disable a whole bunch of stuff, but busybox is supposed to be the minimal utilities needed to run something close to POSIX-compliant, which obviously assumes your system is a POSIX system. It also includes most of the stuff usually provided by util-linux, which obviously won't run except on Linux. If your software is doing something like trying to read from particular device files to get hardware info from the kernel, sometimes that will work on both Linux and BSD (and even Mac since it's kind of a BSD), but it definitely won't work on Windows.

For software that uses the network, how do you query DNS? If you're calling getaddrinfo, I guess Cosmo libc will do some magic for you to make sure that works everywhere, but a lot of software not written in C is doing something not quite that easily made universal, like just assuming /etc/resolv.conf exists and reading it, which definitely won't work on Windows, or using the native platform APIs, which will only work on one platform.

If you distribute desktop software, I would say good luck. Are you going to comply with the XDG? Well, now I guess it will "work" on Windows, but you're not complying with Windows conventions where you're supposed to put stuff in App\RoamingLow or whatever the hell that I would never remember without looking it up every time. Don't try storing data in the registry, because now it will only work on Windows.

Does your software? It better be present on all systems then. Do you query any environment variables? I don't think LC_TIME and LC_COLLATE are provided anywhere but Linux usually. The XDG_ variables are only Linux. Does Windows have PAGER? I don't even know. I'm pretty sure PATH, USER, and HOME work everywhere.

A truly actually portable executable (TAPE) would require far more than an executable file format that can successfully load into memory and execute its first instruction on any platform. There's a reason vendors just write software that uses a browser's JavaScript engine as a platform instead of the OS's native runtime.

EDIT: I don't know if this is ironic, but I guess consider your choice of programming language, too. Go became popular in part because of the static linking, single executable thing, but it inherently can't be cross-platform since it doesn't use the C library and tries to make all calls into the kernel on its own by keeping track of the syscall table provided by every OS it supports. Well, they're still different on every OS, so an executable that works on Linux will not work anywhere else and vice versa.