Everything is always fine until someone comes along and starts selling hosting of Open Source SaaS projects. I cannot talk for Zulip and I truly hope they will succeed with not relicensing or closing part of the software.
Ordinarily I would have gladly used Sentry's community and my good experience with its hosting and customer service as rebuttals but now ...
I feel perhaps you meant to link to https://gitlab.com/gitlab-org/gitlab/-/issues/329596 which describes their intention to accept Sentry formatted JSON payloads and store them in their own database. Searching for "no need" in that issue will highlight the number of times they were drawing a distinction between the effort required to run GitLab and the effort to run the 15 containers of Sentry self-hosted