Once a Worldcon committee has been enfranchised by the site selection process described in the WSFS Constitution, they are virtually sovereign. They have a non-exclusive license to use the WSFS service marks, ("Worldcon","Hugo Award," etc.) and an exclusive license to use those marks to organize a World Science Fiction Convention in their licensed year. As long as they obey the WSFS Constitution, there is no cause for WSFS (through its Mark Protection Committee) to revoke that license.
"Well, what if the Business Meeting unanimously voted to withdraw their franchise?" you might ask. My answer is that no single WSFS Business Meeting has jurisdiction over an individual Worldcon. The Business Meeting could certainly vote to modify site selection so that future Worldcons couldn't be held in a given place, but that can't retroactively disenfranchise a seated Worldcon. Besides, such an amendment would take two years to pass (meaning the ratification would be in 2018 in San José). Furthermore, the WSFS Constitution already has a clause that effectively says that Worldcons may choose to ignore any changes ratified after they are seated if those changes would impose additional financial burdens upon those committees. Requiring a seated Worldcon to relocate or cancel certainly would be an "additional financial burden!"
In the end, assuming they obey the WSFS Constitution and don't abuse the licensed service marks, only a Worldcon committee can relocate a Worldcon. This has happened. It's never been a move to a different country (or even a different state), but it has happened.
I am personally sympathetic to people saying how awful the US government is about "foreigners." Cheryl Morgan isn't allowed into the USA, and she hasn't done anything wrong at all. This isn't recent. It's how the USA treats people at its borders and has been for a long time.
Beyond all of that, let's look at this "move the Worldcon out of the USA because non-Americans can't attend it" thinking more closely. Right now, assuming the unopposed bidders for the next two site selections remain unopposed and win their bids, we're actually looking at an unprecedented situation: three of the next four Worldcons will not be in the USA. (Indeed, they won't even be in North America; there's no Canadian Worldcon in that mix.) I know from my own experience working on non-US Worldcons that there are American fans who insist that they cannot possibly attend Worldcons outside of the USA. It's not just the travel; they say they can't leave the USA, even to cross into Canada. Winnipeg, Toronto, and Montreal were out of the question for them.
Worldcons move around. Some people are privileged enough to be able to attend all of them. (I have been, since 1989.) The movement means that for people with travel restrictions of any sort, be they political or financial or whatever, sometimes you might not be able to go, and sometimes it might be very convenient to attend. For the person who lives in San Francisco and has limited travel range, the 2017 Worldcon might as well be on Mars, but the 2018 Worldcon is within commuter train range, and she'll actually be able to attend.
Am I happy with what's happening in my country? No. Does it mean that "someone" has to move Worldcon? Also No.