I'm looking for an English equivalent of this beautiful, succinct Farsi saying, گر حکم شود که مست گیرند, which roughly means:
If it is decided to get the drunks, they will have to get everyone who is in the city.
It's used when you want to advise someone against going after one person because of a fault they have because you will end up having to go after everyone due to the prevalence and widespreadness of the fault.
English doesn't have a specific phrase like this, but there are many variations of phrases like "if you do X, the jails would be full by noon" or "...the jails would be full by nightfall", emphasising that if you lock up everyone who breaks a law, then you'll run out of places to imprison them.
Examples
From "Six Months in Jail For an 'Offensive' Facebook Status Update", Mark Hillary, Huffington Post, 05/01/2012:
If a government wants to be seen as democratic in this new era of online social discourse then they need to realise, you can't jail everyone who is critical of the prime minister - the jails would be full by tomorrow afternoon.
From a mystery novel, Murder in Murray Hill, Victoria Thompson, Penguin, 2014
Frank had to agree, but if he tried to arrest every bounder in the city, the jails would be full by nightfall.
"let him who is without sin cast the first stone" - implicitly no one will throw then - meaning that the fault under discussion is so prevalent that no one is able to pass judgement
"People who live in glass houses shouldn't throw stones" - this is less aimed at the general public, and is more insinuating that the person adressed is not without fault in this case as well.
If you want to directly render the Farsi saying in English, this might be a more understandable translation:
Going after the drunks would mean going after everyone in the city.
or perhaps
Going after the drunks would mean arresting the entire city.
These suggested shorter phrases have more punch (effect, memorability, ease of understanding).
The following known sayings aren't perfect matches to your presented phrase, thus aren't direct substitutions, but they could work.
You should count the cost first.
Meaning: Make sure, before beginning any enterprise or project, that you can actually afford it, or you may end up paying more than you wish or can afford. Cost, here, can be non-monetary, such as time, effort, or any negative consequences.
In for a penny, in for a pound.
Meaning: This course of action will be hard to exit from later. "In" means "to be entered into at the cost of," similar to asking a people playing high-stakes card games how much money they have at risk, "how much are you in for?" There is a sense of becoming locked in to events or to an enterprise such that a small initial commitment or cost will likely balloon into a large one (or has already), with little chance to escape. Again, while expressed in monetary terms, the concept extends to any kind of cost or negative consequence. A penny is a small unit of money, and a pound is a large unit of money (see monetary symbol GBP).
An eye for an eye will make the whole world blind.
Note that this is only for special situations and isn't a general replacement for your phrase.
Meaning: It isn't wise to take revenge or give out exact retribution, since everyone makes mistakes, and the end result of such a policy (taken fully literally) would result in a lot of eye-gouging, causing many to be blind. Limited to when the proposed action (being likened to cutting out someone's eye) could be considered some kind of retaliation.
A similar sentiment, but more drastic, is expressed by the German saying
Die kannst du alle in einen Sack stecken und draufhauen, da triffst du immer den Richtigen.
(Roughly: "You can put all of them in one sack and hit it — you’ll always get the right one.") Meaning that all of the group of people one is talking about deserve a beating. Wictionary uses politicians as an example.
The emphasis here is probably different: The Farsi proverb appears to indicate not only that the offensive behavior is very common; additionally it suggests that it is pointless to prosecute it. Perhaps it even indicates that it would be foolish. (I suppose that it stops short of condoning the behavior that drinking stands in for, given the official religious prohibition for Muslims.) By contrast, while the German proverb as well acknowledges that whatever offends the speaker is common it still suggests that in principle everybody deserved to suffer for it.
