It will be killed for national security reasons!
Being a collaboration with ESA and Canada it may be more difficult to shelve it, but with a Trump administration scared of every foreign country, who knows? Maybe he'll think Europe and Canada will spy on him using the Webb.... so it will be killed "for national security reasons".
But let's remember the Webb is a very different beast from Hubble. The latter covers from some UV to some IR and can do science that Webb, developed to be mostly a red - to - IR instrument to investigate deep in time and space, can't, so Webb it's not a larger and more modern replacement, it's a different instrument built for a more specific research, a fundamental one, but in many ways less versatile.