The As a Service model is when the software is released under an open source license, and available for anyone to run, while also being made available As a Service by the company.
When the software is primarily consumer oriented, or has a large operational overhead. The goal here is that the software has the simplest on-ramp possible, and requires no effort to maintain over time.
In its most pure form, such as Discourse, the software is always 100% free. That means that, while there is often a single company offering it as a service, the community itself is free to do so as well.
In other cases, the software will be covered under an aggressive form of copyleft, potentially combined with dual licensing, or non-free licenses (such as the commons clause) may be used to create a monopoly on offering the software as a service.
Maybe. With the example of Discourse above, the answer is yes. The community is sustained through the service, but any member of the community would be free to compete. With any model that uses dual licensing combined with aggressive copyleft, or non-free licenses, it trades the fundamental liberties in the core commitment for revenue.