awesome war happening, where to get infor?

You would therefore have to have 13 missions of 100k orc thieves, all of which succeeded, in order to deplete the gold.   That's IF they picked up nothing but gold, which is somewhat unlikely.  They would probably pick up basic resources at least some of the time, so it's more likely it would take a minimum of 21 missions.  If you send 21 separate missions, it's fairly likely that at least one would fail -- and you'd lose 100k t2 orc thieves, with the consequent rebuilding time.

A large alliance could certainly collaborate and do this, but remember, this is what it would require to clear gold from ONE city -- and the player could simply transfer the gold from another city to cover the losses, or from a trade hub.

That's also assuming that the city maintained only an "average" supply of gold.  Most people who chose that particular strategy would likely maintain a larger supply of gold, say 500 million or even 1 billion.  An entire alliance dedicated almost exclusively to thieves would have some difficulty removing that much gold.

In addition, the city could be designated as the alliance capital at any time.  This would give the city's owner instant access to the alliance gold account -- to be transferred instantly into its coffers, with no possibility of blockade.  Granted, this would not automatically work for more than one city at a time (because there are limitations on how frequently one can change the alliance capital), but if an alliance wanted to run one extremely large army, this would be a feasible tactic.

Yes, running negative on gold raises the potential for troops to desert, but for an enemy to cause this to happen is not as easy as one might think.