The expulsion of the Russian diplomats is an obvious act of the Cold War. When Obama already feels neither here nor there, he shows his true attitude towards Russia and Vladimir Putin. What he doubted to do for 8 years, now he manifested his essence at the end of his presidency. Naturally, this rude and unfriendly attack will cause a symmetrical response from Moscow (The Russian foreign ministry has called for the expulsion of 35 US diplomats, Vladimir Putin refused, please read "The Shot" by Alexander Pushkin).
Certainly, this attack is aimed to complicate the potential efforts of Donald Trump’s future administration to normalize the relations with Russia. Trump will have to establish links with Moscow from an even lower level, as it was until now. In general, there is just another series of the "Tales from the Crypt" from the outgoing president.
Diplomats are to be expelled in 72 hours - it is not possible to cancel it now. The new administration is likely not to put obstacles to fill the vacancies. Obama argued that all expellees were engaged in intelligence activities, but this does not mean what it actually is. Of course, in each embassy staff there are always people with "additional tasks", and all of them are usually known by relative authorities. But in such situations, innocent people suffer, and it is very unfair. The current scale of the expulsion simply is to demonstrate the level of hostility to the outgoing president against Russia.
As to sanctions against chiefs and employees of the FSB and GRU, they do not work. The main US sanctions policy instrument is the restriction of economic and financial activity, because the dollar is the world reserve currency. It is unlikely that the GRU and the FSB directors have the accounts in the US banks and related transactions. So here we see Obama’s pure symbolism.
President Obama leaves us. Over the years of his presidency he has accumulated a lot of problems.