There's nothing stopping KPMG from switching to Linux now. The only reason their employees are forced to use non-free software is managers have the normie mindset of "must use Windows/Mac."
Furthermore, when enterprises move to Linux, it's not free as in beer. This is because they need someone to run and maintain their systems. Unlike hobbyists, they don't install Gentoo, they get a contract with a company like Red Hat or even hire their own staff to run it. That's not to say there are no cost savings, Red Hat is probably cheaper than Microsoft.
The difference is when Red Hat is contracted to run Linux systems, if they find a bug and fix it, they are legally obliged to publish the patch as per the GPL. This benefits hobbyists who get fixes and improvements to their free operating system for no cost. The same thing doesn't happen with Microsoft Windows.