Almost every adult in the UK might get a £300 refund from MasterCard

Well, let's see what the settlement number will be like. As it was the case with Volkswagen, the asked amount is extremely likely the amount Mastercard might have to pay. But as other posters rightly pointed out, MC is not the only payment system provider. Will the lawsuit get extended to Visa and Amex? Amex would be the most obvious target as they - albeit smaller market share here - charges the most.
Expecting MC & Co not to make any money on interchange would be silly. After all, without their payment networks the payment work would look very different. They have and will have to invest into technology to keep improving the network. On the other hand, we have an oligopoly and oligopolistic pricing is less competitive than under perfect competition.

@Bob:
That's a fair point. Customers paying by cash are subsidizing card customers. No doubt and the retailer cannot have a dual-pricing policy based on the method of payment.

But if we look at Sweden, it's the other way around. They are pretty much cashless now and cash transactions are more expensive to them. Have the Swedes had a lawsuit against the central bank? I just cannot help to feel that this lawsuit is stuck in the past and behind the curve of progress. My humble 2c.

I don't think the issue is down to the fee charged to the retailer by his processing bank. Rather I suspect it is down to one of the elements that goes into the setting of that charge. If it were down to the fee imposed by the processing banks the lawsuit would be against them.

This particular issue is that one of the costs of doing business as a retailer is effectively down to something called the 'interchange fee'. That is a percentage of any credit card sale transaction and it has to be paid by the retailer's processing bank to the card issuing bank - it is not retained by MasterCard - or Visa, for that matter.

If that interchange fee did not exist, then, yes, there is a chance that prices at the point of sale might be a tiny fraction lower but a Merchant Service Charge (MSC) would still be charged to the retailer by his bank and would still be part of the pricing structure - and I don't think there's any argument over that.

Were it not for the interchange fee, many credit card issuers would have to make hard choices to continue in business profitably. Almost certainly, cashback cards (which pass some of the interchange fee back to consumers) would not exist.

If there were no interchange fee, the business case for being a card issuer becomes more shaky and we could end up with fewer cards, less choice and less competition. Worse than that, we could end up with no interest-free period for full payers or see the re-introduction of annual fees, and that means we all end up paying more.

This is a big deal for the cards industry and it was already rumbling away before I left the industry in 2005.

On the issue of cash customers subsidising card customers, there may be a grain of truth in that but don't forget retailers pay bank charges for banking cash and cheques and that is also built into their pricing structure.