The BSL is a "have your cake and eat it too" license. It allows a company to accept contributions from third parties, without those third parties being able to use the changes commercially. It's immoral.

The moral equivalent of the BSL is to close-source the software, and to offer free-for-use licenses of the closed-source version. They can continue having a fully open-source release, but the release would be changes from 3 years ago.

Sentry isn't doing this because they want the contributions. They want the benefits of open source, without any of the costs associated with it.