I wrote a hard news story about the prospects of the Affordable Care Act (Obamacare) being struck down in 2010, where I predicted that the individual mandate to carry health care insurance would be upheld as a tax. I was right back then and I think it’s even more interesting now, because that’s the reason you can’t really “defund” Obamacare.
Even with a government shutdown, citizens are still required to pay their taxes, and Obamacare is paid for almost entirely by taxes. You can’t “defund” it with anything short of an amendment to the law that gets rid of the taxes (which should send the government back into trillion-dollar budget deficits because you didn’t also get rid of the subsidies/spending), or full repeal.
Republicans don’t have the votes to do either of those things so trying to shut down the government is less about defunding Obamacare than it is about a bunch of rookie Congressmen throwing a temper tantrum over a faux fight that they simply cannot win.