Why did Obama choose to expel 35 diplomats and operatives in retaliation for Russia's hacking during the elections? It appears reducing the number of diplomats may strain our relationship with Russia and make communication more difficult. Or is the idea to replace them with people who did not contribute to the hacking?
Expelling diplomats is the traditional diplomatic move to express extreme displeasure at another country, short of going to war.
For example, Venezuela did it to the US in 2014 for what they claimed was political interference.
Note that the traditional response to this is a tit-for-tat expulsion of diplomats from the other country.
Another example from the Cold War was an exchange of expulsions in 1985 between the UK and the USSR over the defection of a KGB agent.
Expelling diplomats is one of the symbolic things that a government can do to express its displeasure with another government. In this case it has the side effect of kicking out a number of spies.
Russia is a big enough country that they can find thirty-five replacement English speakers to take over any actual diplomatic duties. I don't know that more diplomats in the embassy eases relations between two countries. Many of the people in the embassy will be performing consular services rather than diplomatic services. And it's not clear to me that more spies improves relations. For example, the claim here is that Russia was engaging in espionage activities that harmed relations.
The argument against expelling diplomats like this is that the devil that you know is often better than the devil you don't. The United States could engage in counter-espionage activities against the known spies. And they would have known profiles. The US will need to identify the new spies and learn their profiles. And of course, this tells Russia who the US has identified as spies.
Note that Russia isn't responding in kind. One speculation that appeared on the news was that Vladimir Putin may not want to rebuild Russia's dossiers on the US intelligence team. And of course, he may simply be telling the truth when he says that he'd prefer to just wait until the new administration takes control and negotiate with them.
Another criticism of Obama's actions takes the opposite view. From that perspective, expelling Russian spies is a good idea. The question is why it took so long. It's not like Russia just started engaging in espionage now. People (e.g. Hillary Clinton) were claiming Russian involvement in the WikiLeaks release back before the convention. Why wait to counter until after the election? Earlier action might have prevented some of the later activities, particularly if the personnel involved are being expelled.