This sentence from the Economist is grammatically correct. What "looked like the most obvious route"? Answer: the proposed concept of "raising the ceiling".

Here is the #1 rule for "which": the MODIFICATION must be clear. It's not that "which" can only refer back to the one singular preceding noun. A better rule: you should always be able to tell exactly what part of the sentence is being referred back to with "which". This sentence from the Economist has a clear MEANING.

The reason GMAT books say "which should refer back to the preceding noun" is because the GMAT will often separate "which" from its antecedent in a way that is unclear. For example:

There is a proposed new budget for the health care system, which will finally help us achieve fiscal responsibility.

There is a problem here because we know that LOGICALLY "the budget" is what will "help us achieve fiscal responsibility" and yet the "which" appears to be modifying "health care system".

When you see "which" you really need to focus on logic/meaning.