>In short, this solution from Stripe fills a niche, but it's a very narrow one.
Not too sure about that. Klarna in Europe has a similar system called SOFORT in a lot of countries processing millions of transactions [0]. According to a german article, they processed 2.4 billion euros in 2013 [1]
iDEAL using a similar system (officially provided by banks though) is the most popular payment method in the Netherlands [2].
There is probably a big market for bank transfers with instant confirmation for the merchant. I agree with you though that it is a niche in the US, because credit cards are so common. Expansion in the EU would've probably allowed them to access a bigger potential market.
[0]: https://www.sofort.com/eng-GB/press-releases-sofort-gmbh/pre...
[1]: https://translate.google.de/translate?hl=de&sl=auto&tl=en&u=...