It might reduce the current approximately 0.9% transaction fees on ATM transactions. If you have
a bankcard that has a 3% foreign currency transaction, I doubt if your bank will reduce that.
First, the 3% fee that US banks charge for ATM usage outside the US is totally their own creation. The "Network", which is collectively M/C, Visa, Plus, Cirrus, and a few others, charges your bank approximately 1% to handle the transaction (paying the ATM owner and collecting from your bank), ~½% for the transaction and ~½% for currency exchange. The 1% charged by the Network has nothing to do with the MIF. Major US banks, like Chase or US Bank, who have foreign currency operations, pay the network in euro, therefore only paying the network ~½%. They then turn around and charge you 3%, of which 2½% is profit for them.
BTW, Wells Fargo only charges $5 for foreign ATM withdrawals (about 1% on a $500 w/d). They have dropped the 3% currency exchange fee.
Credit unions and smaller, local banks, without foreign currency operations pay the 1% to the Network. My credit union passes on the 1% the paid and then charges me an additional $1 for using the foreign ATM. I used to use a local bank that charged an additional 1% (2% total) plus a couple of dollars.
Banks like Capital One, which have no charge for foreign ATM usage are absorbing the ½% transaction fee as a cost of servicing your account.
Changing the MIF has no effect on ATM transactions.
Point of Sale (POS) transactions are entirely different. For every dollar you charge on you credit card, your US bank (through the Network) gives the merchant 96 cents. That 4% they keep is called the "Interchange fee". It's out of that fee that they pay you travel miles or cash back. When the EU limited the Interchange fee that European credit card could charge European merchants, they exempted US credit cards from the limit. Apparently, now, they have used a little political arm-twisting to get the US CC companies to lower their Interchange fees. This should lower a little the price everyone pays for goods in Europe, but it will not directly affect us noticeably. However, it might eventually change the "rewards" credit card companies give for transactions in Europe.
One European train company already refuses certain credit cards
because they have a very high interchange fee
I'm not sure which rail company that is, but I know a little over a year ago, when I purchased Sparpreis tickets from the Bahn using my US credit card, I paid a small surcharge (a few percent, not 4%) on top of the fare (plus 3% to my bank for currency exchange).