LD_PRELOAD is also useful for working around hard-coded file paths inside of closed source (often commercial) software. This is how we make some closed source software work on NixOS, where we need a way to redirect file IO to corresponding paths in the NixOS file system (when those paths aren't the same as on typical distros).

That use of LD_PRELOAD by Nix is NOT a downside.

But it is not a good workaround for Nix thus making Nix a poor candidate for a reasonably secured production servers.

There are several others mentioned so it shouldn't be a default but none of those use cases apply to normal users otr prod servers.

yeah, the following positive usages of LD_PRELOAD is enough to cobble together and hijack a system, from userland.

https://github.com/gaul/awesome-ld-preload

Also LD_PRELOAD gets used by Debian `fakeroot` for building packages.

So no can do disabling LD_PRELOAD on Debian (much less be able to use libmusl on a Debian)

Most people though will not be building packages.

At least the Gentoo folks, not withstanding.