Linux

I think people are more likely to give up the security of not having constant admin access if they could get the same software they do now.

As I said in the wall of text above, going "oh, by the way, all your software won't work, but you will find alternatives" is a massive dealbreaker. It's one thing to bring people over to Linux, but trying to do that AND transition people to a full open source environment is a crazy overreach. A Linux distro focused on being Windows-like by offering closed software out of the box and better options for a click-and-play Windows compatibility layer would be way more appealing to many. You can work on their open source bona fides once they are inside the ecosystem.