It tends to be a spectrum, but the various, sometimes very different licenses adopted by the OSI still have something in common - the open source values laid out in the Open Source Definition. [1] SSPL did not comply with this requirement, as it discriminate against specific users or use cases. I think it is in the interest of everyone to draw the line somewhere.

[1]: https://opensource.org/osd/

Open source licenses might be a spectrum but it's likely that the legal reality isn't. Like the famous double split experiment there's probably no spectrum and only a couple of buckets of legal validity.

As far as the buckets go, I still believe there is a pretty large spectrum.

BSD vs GPL vs LGPL are pretty dramatically different, and there are many in-between that might satisfy the definition of “open-source” at face value.

BSD would not be OSS by this definition, as it has no clause against discrimination of people.

OSS is IMO a very subjective term. There is a massive gray area surrounding open-sourcing derivative works, etc. and other stipulations.