Yes and no... 0.3% is the cap of the interchange fee of consumer credit cards, i.e. what the card network pays the issuing bank. There's no cap on corporate credit cards, which granted isn't relevant for those collecting points on private credit cards, but regardless, the cap is only on one step in the fee chain, there are two more:

• The fee between the card network (like MC and VISA) and the merchant acquirer (often local brands, but there are global acquirers like PayPal and JP Morgan).
• The fee between the merchant acquirer and the merchant.

The last step is what you're actually paying extra for at the merchant and that hasn't necessarily come down much after the EU cap on interchange fees. Of course it depends on the bargaining power of the merchant. If I personally want to accept PayPal payments I need to pay 3%+, take it or leave it. However, if my name is Zara or IKEA, of course I can negotiate a better deal with a merchant acquirer.

Your point is of course that for consumer cards, European card issuers only have 0.3% to play with for card benefits, anything above that will have to come from consumers paying exorbitant interest rates on revolving credits. My point is that regardless of that 0.3% cap, the consumer still pays a lot more than that to the merchant, simply because there's no cap for what the merchant can be charged for accepting your card.