A Scalia elevation to Chief Justice has several advantages:
1. It does not change the ideological balance of the SCOTUS.
2. The position of CJ is not a powerful one, but more a first among equals position. You will not be strengthening a convervative court by having a conservative JC.
3. It gives the Democrats a chance to look non-partisan. They can say, "Look, we didn't filibuster this conservative Scalia; we're not really against all conservatives, just the extremes one like [insert name of Bush nominee]."
Deciding not to filibuster would be a PR advantage for the Democrats.
The question is, will Bush nominate him.
You give the Democrats way too much credit, J.J.