Currently there is no convenient way for end end user who has purchased software from an ISV to install a side-loaded MSIX package machine-wide (for all users). Software is often licensed on a per-ma...
In case you are not aware...
1) MSIX Installation may be performed in powershell using the Add-AppXProvisionedPackage cmdlet. This methods provisions the package in a way such that any user logging into the box will have the installation registered for them. This might provide what you need for the "all user install" scenario. This functionality is also available from DISM in case you are thinking about image management.
2) Supposedly, there is group policy support today to prevent uninstalls. I'm not sure if it is AD or AAD only. I think that today the control is a hammer affecting everything, but John mentioned more granular controls in the works in his recent video.