I’ve already written about how I think whether a SCOTUS Obama nominee gets a hearing and possible vote depends upon whether the controversy looks like it is causing trouble for Republican Senators in blue/purple states up for reelection. If they face pressure, and McConnell can lose the Senate, I think Republicans will relent.
But here’s a more extreme version of this scenario. Suppose Obama nominates a very strong candidate who is a moderate, like a Sri Srinivasan. Trump is the Republican nominee and Hillary Clinton the Democratic nominee. Trump looks like he’s going to lose (or perhaps he already loses). Clinton signals she will nominate a liberal Scalia or second Brennan (think Pam Karlan) after inauguration. The Senate might even flip (or be about to flip after a lame duck) to Democrats, who can kill the filibuster.
In that scenario, it is completely in McConnell’s interest to allow a vote (and perhaps even vote for) the Obama nominee. The alternative would be worse.
(Now if it is Clinton but it looks like it will still be a Republican Senate, the incentives and play are a bit harder to imagine).