Members of Congress have long worked together on fulfilling key functions of the government, even when it's not in their party's best interests. It's the United States Senate, after all, not the Republicans Only Senate. Discarding these customs in favor of purely partisan might-makes-right governance leads us to some very unpleasant hypothetical scenarios. For example, why stop at not allowing the president of an opposing party to make a SC nomination during an election year? And why stop at SC nominations? Suppose that Trump wins reelection, but Democrats take the Senate. Would you be okay with the Senate then saying that they will refuse to hear or vote on any nominations that Trump makes? No SC justices if any vacancies appear, no new judges, no new Cabinet members, nothing. Or when the time for passing a new budget comes, Congress refusing to pass a new budget until Trump and Pence resign? They have the power to do either of those things. Nobody's going to stop them.
Republicans won't be in power forever, and Democrats won't cling to their principles for much longer if Republicans continue to play dirty. It's in the interests of both parties, as well as all of us regular people, that they abide by some general rules and customs that establish a certain level of consistency in governance. Setting the welfare of the nation aside to focus on kicking the shit out of the losing political side - and then switching positions every two or four years to do it all over again - is no way to run a country.