Gesture to appease Joe Public
Expelling known spies is and has always been just a gesture to show Joe Public, "Look, we're doing something about it." Just political bullshitting, to be honest.
Much harder to expell spies the US doesn't know are spies. Even more difficult to expell those who have an American passport. And those are the one to worry about.
On a side note, I don't buy this RU interference nonsense. It's a desperate attempt to depict Trump as an illicit successor in the White House. (Disclaimer: I think he is a shite candidate. But so was Clinton. Choosing the lesser of two evils was particularly hard this time around.)