I think it is great Sentry folks do not pretend it is Open Source. It is great to see innovation and we will see if it becomes popular.

What I see is a lot of "single vendor" projects which carry most of development burden want their business protected while having, often minimal, contributions from community.

The "Real Open Source" though is multi-vendor - think PostgreSQL, Linux, Kubernetes among others, in which case many parties share the burden of development and maintenance but also have even playing field monetizing product.