It varies based on the card type, country, and other things. We could expose all of that more directly but the trade-off there is obviously pricing complexity, which we'd like to avoid.
In our analysis in making this change, we saw that this simply makes little difference for the vast majority of businesses. For example, if you're processing $100k/year and refund 5% of your payments (which would be on the high end of normal), it works out to about $12/month.
While refund rate of course is not definitively coupled to the quality of a business, we do see across our portfolio that it is strongly correlated. Given a basket of possible fees (for example, higher fees on Amex, which most other providers have), we prefer the fees that, on the margin, are least consequential for the businesses that are doing the best job of serving their customers.
Having said all of that, none of our pricing is cast in stone, and we always genuinely appreciate feedback, including contrary views.