One would assume that if paypal authorises a fraudulent Visa card for example, that they rather than the issuing bank would now have to eat it, and consequently would raise rates to cover that risk. Of course given that paypal 'banks' under Swiss law, that may already be the case.
I still believe they will be cheaper than any card merchant unless you are very high volume.
The paypal users that could get hit are those that use the virtual terminal and take card details by phone etc. without the cardholder present. I never signed up for that. Because I wrote my own paypal shopping cart I am able to generate a 'cart' for anyone to enter in their own card details, passing the buck back to paypal to take responsibility for the security checks. I think it costs me a tiny percentage more but my volume means I only generally pay 1.9 or 2.4% which I still think is very fair for being able to take virtually any card from any country.