ISS was actually supposed to be deorbited in the 2015 timeframe under Bush II (to make funds available for the Constellation program), later extended to 2020 and then 2025.
I think most expect ISS to stay in orbit until 2028 or maybe a little later, so some extension is likely to happen regardless of what Trump's policy is (same as how Obama extended ISS). 30 years is a really long time for safety-critical space hardware, and ISS absorbs an astronomical amount of funding. It's not hyperbole to say that the Trump Moon shot would have plenty of funding if ISS wasn't around. We're talking about an additional $4-5 billion per year.