I have been reading the forum and can see that I'm not the only one that is looking for alternative payment gateway support for XF! For more than 3 years people seek support for the major billing companies like 2checkout, BitPay, Stripe, Skrill, Payza and so on.
You should really consider to get this integrated.
PayPal is not the only way people pay on the Internet.
To give an example using a new system, we're currently spending time building a generic payment framework. This is designed not only to allow different payment processors (not just PayPal) but also to be a framework to make it easy for new "purchasable" elements to be added easily (not just user upgrades). A system like this benefits the core system, but also enables new add-ons to be developed more easily. Without this, developers may discount an idea that requires payment processing because of the extra legwork required.
What payment provider? Some providers offer only a primary creditcards, while others also offers more than 100+ different online payment methods. Especially in Europe there is a huge fragmentation between countries who have their own online payment system. In these countries most people do not have paypal or a creditcard.
Only Creditcard like and or Paypal payments is too limited.
Unfortunately, some do not trust Stripe etc. A lot of customers don't understand Stripe or Braintree (my preference is Braintree, btw) working in the background. They can think it's insecure to not see the PayPal symbol and this could lead to a lack of trust for small businesses. PayPal support is definitely essential for an online business in my opinion.
They should contract multiple payment providers and choose the one who offers the most payment options for each country. Many Chinese online shops use this method. XF should focus on local payment instead of credit card or paypal only. Also the option to buy license anonymously, still not a option?