There's no reason that Chargebee, Recurly could not be emulated in open source software. The solution to the saas problem is simply a nice API + dashboard, with pluggable billing backends. You should be able to swap merchant accounts seamlessly, since that's the biggest point of failure.
What would be nice is a "launch KillBill on digitalocean" button (I think someone posted a Show HN like this recently?) so that you could launch your own service and start using it immediately.
More generally, I think a lot of SaaS startups are ripe for disruption from open source projects. However, many projects miss the mark of usability. What people need is a self-hosted, open source API that's as usable as chargebee or recurly, for instance. This means that OSS should be targeting microservice architectures more often, instead of self contained solutions with a bunch of wiring required. I want to see more OSS projects that are structured as a standalone app server with an API and dashboard, so that you can self-host but still get the benefits of a composable API.
A good example of an open-source project leveraging "microservice architecture" for self hosted solutions is Seatgeek's AB testing project, sixpack. [0]