The real question i see nobody addressing is this. Why are all liberals so insistent on appointing a new justice before obama is gone?
Obama has almost a full year until the end of his term. If he were to agree to the argument in delaying his appointment, he'd be agreeing in deferring all major decisions until next year and would set himself up as an early "lame duck" president for a full year.
The argument might make sense if the vacancy had opened up after the election, but to agree to the Republican's demand now he'd be agreeing that he's lost the authority to make major presidential appointments and decisions for the whole year.
I'm pretty sure that's the political reason. Judicially, though, it's unheard of for the Supreme Court to go so long understaffed. It'd be setting all the wrong historical precedents. Fully two thirds of the US government would be weakened.