MIT license is still open source, so Ubuntu based operating systems can still be open source. The problem is that this makes it less needed that they have to be. However most current projects will probably stay proper open source projects and likely continue to use a better license.

You can however Switch to some open source software without having to go full Linux. Introducing Libre Office, Firefox, Thunderbird and so forth can be done in Windows. Software Not working one Linux is often a big problem for switching.