Consider almost every OEM install system likely ships with exactly one OS installed. So that’s all the Dell, Entroware, System76, Tuxedo and many more. Then look at the data for clean installs. “Install alongside” is less than 10%, and that’s only a proportion of total users, given the pre-installs mentioned above.
Further, corporate desktops tend to be single OS, and Virtual Machines even more likely to be single OS install.
I’m not saying this is conclusive proof, but it’s data we have which points in a direction.