Nailed it.

It's a though situation to be put in. But dear OP, while I understand you're trying to come from a rational POV, you let your emotions - namely fear - get better off you. Also, you don't seem to be very well informed about lives and possibilities of people living with Down Syndrome. So all I read here is "I'm scared to be a dad to a kid which demands a lot of attention. (Newsflash- all kids do. Even healthy ones. And even without genetic disorder there still could be many other unforseen issues.) I'd rather put my energy and means into other stuff. Now I'm giving my wife an ultimatum to do as I say with her body or I'll walk away."

You have every right to walk away and provide your bare minimum the law requires you to. But don't you think that maybe a better approach is to admit that you're scared and ask for help?

Therapy, both individual and couples could make it easier for you two to understand where the other is coming from. Meet some compromise, but do it together. At least in along run it'll help you to mitigate any resentment towards yourself and your wife and kid (if she chooses to keep it).