Also, there's no real hole to plug here,
Well, there isn't because Trumpism is a natural consequence of Republicans' rhetoric of the last several decades. Its rise was an increasingly distinct possibility, but I think one that was horrifying enough that nobody seriously considered it. Don himself is largely incoherent but his platform, such as it is, is cut from the same cloth as the likes of Cruz and Kasich. He just added a huckster flair for showmanship and a willingness to ignore any and all of the outer bounds established by 'conventional wisdom', and
damn did he make it work. It's not a
winning strategy - he's never been a very viable candidate; polls have had him trailing Clinton for the entire campaign and that has only solidified as we approach November - but it's
close to winning, which is enough that now that it's proven out it will be used again in some form. He's proven just how far a person can go and still retain a large base of support, or conversely, his campaign is the exact kind of thing that gets a large part of the Republican base
really enthusiastic. We don't need to worry about theoretical crystal-energy-Hitler candidates in the future, just another Trump.
How is Trump would be usurper?
Relying on the assumption that putting Trump in the White House would be a Very Horrible Thing for the foundations of American democracy (his fascist tendencies, ties to Putin, etc.). A very reasonable and probably correct assumption, in my opinion, but an assumption nonetheless unless one of you is hiding a time machine.