And Windows is used on business PCs largely because of how manageable they are at scale.

... Linux being manageable at scale is kind of the reason why Linux is the standard for servers. Many enterprises run Linux workstation distros, and they can be managed at scale just fine, it's just different tooling. You can deploy a Linux desktop OS with Ansible as easily as a Linux server.

You can replace pretty much the entire Office suite with Nextcloud and OnlyOffice, both of which can be easily hosted on-prem, for a fraction of the cost of paying MS for roughly the same thing on their awful infrastructure.

If it was feasible for business to change to a free alternative, I guarantee they would've done so.

They have. Just because you haven't heard about it doesn't mean it isn't happening. It's pretty easy (and inexpensive) these days to run Linux desktop OSes like RHEL, Debian or Ubuntu on a VM running on Proxmox or OpenShift, complete with multiple monitor support and GPU. Hell, you can even run a Windows VM if you want. All you need is a system (like a thin client) with enough grunt to run a browser, and enough ports to handle multiple monitors and USB accessories.

And businesses aren't interested in "free", they're interested in support, which they are willing to pay for. This is how companies like Ubuntu, Red Hat and SUSE make their money. The OS is free, but you can pay for professional support.