Tuesday Thought: Which--as the subject opening a sentence

Which normally operates as a relative pronoun acting as the subject of a relative subordinate clause, as in this sentence from a New York Times Sunday opinion on February 8, Bank Bailout, Redux:

But the pay curbs are minor in the grand bailout scheme, which will ultimately put hundreds of billions of tax dollars, if not trillions, on the line.

Normally the relative clause is set off by a comma to signal that it is commenting on a noun (scheme), not defining one. But occasionally it is set off by a dash--or even a period, as in this sentence, from the same opinion:

And such guarantees were presented as generally low cost and low risk. Which they are, until they aren't.

Following that sentence is this one, opening the next paragraph:

Which brings us back to the huge sums it will eventually take to repair the banking system.

Here, I would argue, which is no longer relative, because it lacks a clear antecedent, but substantive. I'd replace it with that.

That brings us back to the huge sums it will eventually take to repair the banking system.

So, the opening which operating as a substantive should always be replaced by that.