In this situation, "what is Mr. Big planning" certainly fails. But is that the best question to ask? Are these plans vulnerable to "what is the most vital sub goal in Mr. Big's overall plan that is actively being pursued"?
Note that if Mr. Big explains all details of his plan to Fat Tony, once the PCs capture Fat Tony they know the entire plan. So Mr. Big needs to keep his information spread out, meaning he can't afford to lay out the entirety of his plans in any single meeting. Why is it easier to find an hour in which Mr. Big explains all of his plans than it is to find an appropriate question for the oracles?
Because appropriate questions are very hard to come up with and the answers are very much up for interpretation.
It is cheaper than scry so they could use the ritual multiple times to refine questions and follow up on answers. But it is hard to adjudicate the fine line between plot breaking reveals and useful expenditure of money.
With a reasonable scrying spell you can give useful information and not have a chain of follow ups getting to the plot breaking questions. Do a follow up scry and oh so sad the meeting is over now.
Don't get me wrong i can probably stretch out consult oracle to a lame level, but its small amounts of money to break the plot.
Lets say the players know enough to ask this "what is the most vital sub goal in Mr. Big's overall plan that is actively being pursued?
I can say takeover
follow up in Mr bigs most vital sub-plot what is he planning to takeover?
The utilities
Etc.
But still as slow as it is dished out more money can get more and more questions answered, and eventually you reach plot break. So you either get ambiguous only somewhat helpful answers as I think oracle was intended, or it breaks the plot.
With scry which costs more you get basically nothing, you can maybe find someone if someone who you are willing to drop 20k+ in GP on is not willing ot defend themselves against scrying and that is about it. Though I suppose you could dodge a lot of these things by scrying for something like the Mom of the person who murdered the king. Which brings us back to plot breakers.
If it was a somewhat effective spying spell you could use it to get a wide variety of information but not break large plots with a single spell. Further more with a longer duration you can use it against targets that you would not expect to have scry protection and still get some useful information.
what they really needed was a decent duration scry spell with limitations on the target, like you have to actually have some specific knowledge of who they are, not generalized who killed the king stuff. Then its just an alternate method of spying, and providing that is a good thing since having a ninja on the team should not be required. Basically they gave us two choices useless rituals or break the plot ritual, I don't think either of those is a good thing to have in a game.