Dark pools mean that public market prices aren't as accurate. The story of financial markets being valuable to wider society is, among other things, price discovery. Dark pools don't serve that purpose.

Moreover, the PFOF story depends on 'price improvement'. If public stock markets aren't accurate, then 'price improvement' is a meaningless concept, because it compares the price to the public stock market price.

> It’s no different than telling your neighbor you’re willing to sell your house for $X without publicly listing it.

The difference is that the most accurate price for 'my house' is not of significant public value. Whilst the most accurate price for stocks is quite valuable to the wider world.

It’s worth pointing out that, in most U.S. localities at least, real estate transactions become public knowledge once they close. You can go look up, on a government website, how much a property sold for even if the transaction was conducted privately. So price discovery is still possible.

I don’t think there is an equivalent public reporting mechanism for private “dark pool” transactions of financial instruments.

Another way to say “without impacting the market” is “denying the market information it needs to set prices accurately.”

Also: what's "monetary" about them?

Assuming you mean OTC desks, because there isn't much proper dark pool presence in crypto. OTC desks aren’t where majority of spot and delta one products trade although some of their advertising might have you think otherwise.