First, letting all the cuts expire would be bad for the economy. Monetary policy up against the lower bound, contractionary fiscal policy, blah blah blah.
Second, he'd sign a new tax bill without cuts for the highest rate? Passing it through a GOP House, where the GOP leadership will introduce it after it's been voted on by a GOP-majority committee? Then in the Senate, where Republicans would break ranks to get it past the filibuster?