> The DX of Stripe is already great

This used to be the case back in 2015, but not anymore. The financial compliance is more strict now. You have to charge taxes. EU enforced SCA / 3DS in 2019. All of these are hard to implement (correctly) on their own - almost impossible together.

Source: I run (paid) Ruby on Rails library for Stripe subscriptions integrations. I also do billing audits. Here's an example audit where I pay $30, get ~$2000 https://www.youtube.com/watch?v=YuXp7V4nanU