However, they have repeatedly made statements that they are committed to open source- some only months before this announcement. The 11 year old decision argument doesn't hold true.
They have a right to change their mind. But they shouldn't be lying to their customers and the community now that their mind has changed- https://sentry.io/_/open-source/ They continue to advertise that they are open source on their marketing website. That is a lie, and it marks Sentry as an unethical, dishonest company.
The wider debate on licensing terms and business models can move forward- it has existed since the beginning of the software industry and likely will persist for the foreseeable future. I prefer open source licenses. I think they are the best proposal for software licensing and distribution so far. If Sentry wants to do something different and use the BSL license, that is fine- but they shouldn't lie and claim to be open source.