I've never seen them say it explicitly, but every article I read seems to be along the lines of "physicists thought/hoped the standard model could be complete,

I've never seen them say it explicitly, but every article I read seems to be along the lines of "physicists thought/hoped the standard model could be complete, but then this little particle came along...". Even the fact that they think they can have a model with a finite number of particles gives the impression they think this is more likely than not to be the case.