In this case, it’s likely that they have set up recurring SEPA debit payments. Changing the payment provider won’t keep the revenue coming in - it would require every customer to sign up for debit again. And when stripe figures out the mess, you need to consolidate the two providers or you’ll end up debiting twice.
Gumroad has their own failures, but if ANY issue with payment processing is intolerable, don’t bother running a business.