While I believe the immediate announcements are token and positioning (the more obstinate they appear, the better the odds of a more moderate candidate) it doesn't make me LIKE it any better.
Which is why they are likely to put up token opposition - enough to placate their supporters. Everyone panders to the center when election time comes, because that's where most of the voters are. This is why the rhetoric is extreme during the primaries, but is significantly toned afterwards.
This isn't unprecedented. Obama is a lame-duck President facing a majority opposition party in both Houses of Congress. His influence is close to its nadir. Scalia is a lion of the GOP. It would have been hard to replace him in 2015. It's probably impossible in 2016.
The GOP is making Scalia's replacement a campaign issue. So be it! If the Democrats win, they'll have earned a mandate to replace a lion of the Republican Party with a committed liberal.