They didn't say "simple", they said "simpler". If we are talking relative complexity between SAP/Dynamics and Stripe Billing, yeah, billing is "simpler".
Stripe's own claim to fame and much success is making an exceptionally complex topic, like billing, seem simple. It never is. I would imagine most of their own customers have far simpler needs than they themselves do.
FWIW, we're happy Stripe customers, but also candidly do much of our billing outside of of them. Some of the examples GP cited are in a similar boat. Which is all to say that Stripe solves a lot of headaches, but it hasn't solved all of them. I'm sure investors in Stripe see that as a positive opportunity.
If you could wipe out the past and start from day dot then /maybe/ you could transition without too great a risk; even then, you’re looking at a massive transformation project with a lot of temporary contract workers. It’s expensive, because billing fundamentally touches every part of a business - if it breaks, you can’t pay your service providers and you can’t get paid by your clients. Optics doesn’t matter here, dogfooding is a nice bonus and not a given. There’s just no compelling reason for them to move over.